Playing chamber music is essential to learn basic music skills. 
When you play chamber music, you are forced to develop your musical instinct and listen to your colleagues to feel the rhythm and tempo or take the lead yourself when it's appropriate. By having rehearsals without a conductor, you learn to create your own interpretation of the music and solve little problems (intonation, rhythm etc.)  on your own. When you do so, it is important that you respect everyone's ideas so you don't lose your friends in the process! When you go back to your band or orchestra, you'll be able to follow the conductor without depending on him/her. 

If you are in school, talk to your friends and see if they are interested in playing in a group with you. The most common formations for brass instruments are:
Brass quintet (2 trumpets, one horn, one tuba, one trombone)
Horn quartet
Low brass ensembles 
trumpet quartets
brass trios ( trumpet, horn, trombone)


You will find quite a lot of music for these formations.  Some pieces can be downloaded online for free but it's worth it to spend a couple of bucks to have more choice and get a nice score. 
A lot of music publishers will offer alternative parts in case you don't have all the players you need. ( ex: there can be an euphonium part as substitute for trombone or horn etc.) 

Get some coaching if you can and look for opportunities to perform in a concert. It can be a student class recital or you can even play a number in the band concert. 

Before we start this topic, it is important to say that playing music is not an olympic sport. You won't get a medal for playing louder than your colleagues in a concert. In fact, playing louder than everyone else is a rather rude and selfish thing to do. Remember: it's not about how loud you can play, it's about how loud you should play. With that in mind, open your ears and use your artistic taste to determine your dynamics. Don't let your physical ability to blow in your horn dictate how loud you will play. 

Having said that, when the situation demands it, you need to be able to play a good forte or fortissimo. Developing a good dynamic range will take time and patience but if you work on it daily, you should see some fast progress. 

Here are a few things you can do to make more sound:

1) long tones with 8 beats crescendo and 8 beats diminuendo. Doing a few of these daily will help your tone and increase your dynamic range.

2)Breathing exercises. Take deep breaths and stretch your harms at the same time making a "T" shape with your body. This will expand your rib cage and increase the air intake. Immediately pick up your instrument after a few breaths and play something. You'll be amazed how good it feels.

3) Fool around a bit. A few years ago, I asked a colleague who could  play super loud how he did it. He said that he just spent a few weeks playing ridiculously loud just to fool around and that it worked for him... 

4)Embouchure building techniques. If your chops are stronger, you will be more relaxed in the high register and will get a better air flow to make more sound.

5)Don't be shy! Play with confidence. Go for it and don't worry about missing some notes. Your audience wants to be entertained. They don't mind you missing as long as you really try. Fear of missing notes will make you hold back and you won't be able to play as loud as you want. 

I'm sure there are a lot of other things you can do to turn up the volume but these 5 ideas should make a difference. 
Have fun!

You will soon or later be asked by a conductor to play at an uncomfortably soft dynamic. It may be because you are accompanying the strings in the orchestra or simply because he/she wants it that soft. It can be a very unpleasant experience if you aren't prepared for it. 
To avoid the embarrassment of not being able to play a very simple passage in front of everyone just because the conductor wants it too soft, you have to take a few minutes once in a while to practice at a ridiculously soft dynamic. Play a scales in whole notes very slow as soft as you can even if the sound stops sometimes. You can also do some simple etudes ppppp for a few minutes. Long tones are also good. 

I know it's not the most exciting thing to practice but it will save you a lot of trouble some day. Also consider that the softer you can play your pianissimo, the louder your fortissimo will sound because of the contrast you'll be able to create. So it is musically very useful to have that "trick" in reserve. Practicing soft is also very good to relax your chops when they're tired after a day of playing. 

You should always play musically especially when  you are alone in your practice room. This is the place where you can really let yourself go and try things. No one will hear you so go for it! You will be able to play musically in concert only if you practice this way. If you never take any risk in your practice room, will you take some in concert? Most likely not. So don't wait for a miracle; make it happen right now and you'll be able to do in in a performance. 

To play more musically, you can imagine that the music you play is film music. Create a scene for every phrase you play and try to "accompany" this scene with your instrument. Make a whole story out of one piece and and use it to inspire your playing. Play the story rather than the notes. Of course, if it already is film music you are playing, you should have no problem finding the story!

Imagine the melodies you are playing are opera arias. How would a tenor or a soprano sing them? Sing your music in your head like a great singer would sing on stage with great power, energy, confidence and musicality . Then try to copy this with your instrument with the same character. Don't be afraid to make a big show! You can try anything you want when you're alone in your practice room. It'll probably make you miss a few notes in the beginning but it'll greatly improve your musicality and tone and once you get used to it, you'll miss fewer notes. If you practice this way, you'll be able to perform this way and you'll have lots of fun playing!

Playing music isn't just about playing your own part. It's also about listening. When you play in a group, don't focus on your own part. Listen to the entire ensemble and you will hear yourself in it. If everyone does it, the group will automatically sound better. When I conduct small groups, I tell the students to do that. It's amazing how the intonation and ensemble immediately improve.

Don't be just a good player, be a good listener! 

I often say as a joke that if someone's evening is ruined by me missing a note or two in the concert, I'll gladly reimburse his ticket! 

Seriously, I use to be upset after missing a note in a concert or even in rehearsal. With time, I learned to accept that I'm not a machine and that missed notes are just part of the show. Of course, no one misses notes on purpose but if you just focus on that, you get more nervous about it and end up missing more.

There are a few things you can do to improve your accuracy though. The most important is to hear the notes clearly in your head before you play them. Try singing your music with your voice before playing. Take your ear training class seriously! We often say, if you can sing it, you can play it. 

Being in good shape and having a good feeling in your chops will also help you. If you get tense, you'll miss more notes. That's why I said earlier that you shouldn't worry about it too much. You'll end up in a vicious circle of worrying-getting stiff-missing more-worrying more etc.

Be patient when you learn a new piece. Your brain and body need time to develop the right reflexes for each new piece or etude you play. If you allow yourself proper time and don't expect perfection immediately, you'll improve gradually and will gradually miss fewer notes.

When I go to a concert, I want to hear great music, musicality and energy from the players. I don't really care if they miss a bit as long as they try to make something interesting. I tell my students not to try making something perfect but something beautiful. The purpose of music is not to hit all the notes but to touch people's hearts. 

Next time you practice, free yourself from the obligation of hitting all the notes. You'll still miss a couple but you'll sound much better, your chops will feel great and you'll have a lot more fun playing.

You can work on your dynamics, articulations and phrasing by practicing a phrase on one note. It can be any note you feel comfortable with. You can  for example play the whole passage on a B flat if it's the first note. It will sound a bit silly but it really works. 
When you practice on one note, you don't have to worry about hitting the notes and you can focus on all the other things that you need to play musically . Try to make the phrase interesting and musical this way and then play it with the right notes. You should have a much nicer phrasing and sense of direction to your phrase after that. 

There will always be many different opinions about how we should play technically and artistically. One teacher can tell you to play in a certain way while another will tell you the exact opposite!. On top of that,if you search the web, you'll find an unlimited amount of ideas not always going in the same direction. So how do you sort out all this input? Who's right? 

You can trust one teacher or another but the person you should trust the most at the end of the day is YOURSELF. 
You are the one who's playing and you're in the best position to see if what you're taught actually works. You have every right to seek as many opinions as you want on a specific topic and choose the one that YOU find the best. It's YOUR playing and YOUR life. You're the boss.

Before you ask a teacher what he/she thinks, you can ask yourself what do YOU think and how do you feel about it. At the end you're the one performing so your opinion should matter. Make your own experiments. Try things for yourself and be your own teacher. Some things you'll try will work and some won't. That's okay. You don't need a 100% success rate. You'll always make more progress if you're proactive about your playing than if you just passively wait for someone else to show you the way.

As long as you regularly get feed back from a teacher who has more life and playing experience than you and can make you see and hear things you wouldn't notice otherwise and show you techniques and ideas, you can make your own decisions and be in control of your own playing.