This is a quite easy - yet very important - trick: it will be the start of almost every other trick, so it's important your dog learns it well.
Quite simply, the whole idea is to have your dog sitting...
How to do it
There really are various ways to do this: this is one of them. Stand in front of your puppy and hold a hand above his head: say 'Sit' and move your hand slightly behind the puppy's head. The puppy will raise its head to watch your hand and his rear end will lower. When the puppy's rear begins to go down, say "good sit." Repeat 3 or 4 times or less if the pup loses interest.
Tips & Tricks
For older dogs you may need to "help" the sitting position by lowering - with a delicate push - the rear of your dog. Always cheer him as soon as he sits - and in a few repetitions your dog won't need any "help" to sit.