When you single tongue, you attack each note separately making a "tah" attack. With double tonguing, you will alternate between "tah" and "kah" attacks to play faster. It takes a bit of time to get used to produce the "kah" in the back of your mouth while playing. You can practice scales just on "kah" attacks to strengthen your muscles back there. In the beginning, your tongue will get tired quickly but it should improve with time.
First practice with only 3 notes. Repeat "takata.... takata..." many times. Then add more notes to the sequence until you can do "taka taka taka taka..." non stop comfortably.
For triplets, use triple tonguing. Instead of doing TKTKTK, you make groups of TKT and repeat as much as needed. You can also do TTK for your triplets. I personally prefer the first method.
Before you practice on your instrument, practice "speaking" your double or triple tonguing.
Say "taka taka taka taka ta...." for double and "takata takata takata...." as fast as possible for triple. This way you can get used to it and add some speed before you play it.
Here is a little video about double and triple tonguing to give you an idea. There are more videos with slightly different opinions about it. Some people argue about "takata" vs "tukutu" or even "tagataga..." I think it's a minor detail. You can try different options and see what works the best for you.