Every man should know the basics of training a dog, and at the end of the day it's not too difficult but it does require time, patience, and dog treats. The key is to figure out how to make the dog do what you want him to do without forcing him, then it's a matter of reinforcing the behavior with a command and a treat.
When training a dog to sit you'll need a leash on him. The easiest way to get him to sit is to tug directly up on the leash. Stepping towards him may help as well. Be gentile when doing this, the goal is to get him to sit on his own accord, not force him to sit. Forcing his butt the the floor will not teach him anything other than you're his alpha, he should already know this. When he sits tell him the command you wish to use, reward him with a treat, praise him, and pet him for being such a good boy. Then repeat until he sits on command. Be patient and don't get frustrated with him, he's your best friend after all.
Bonus lesson: get him to shake hands! Now that we've gone over the basic concept of how to train a dog it should be clear to you that the key is to get him to do what you want him to do on his own accord. So, how exactly do you get a dog to put it's paw out? It's easy, put a small treat in your hand and make sure he knows it's there then make a fist so he can't get it. Move your fist close to his nose, he'll probably start lick your fist but eventually he should get frustrated enough to start swiping at your fist with his paw. This is the behavior you're looking for so reinforce it by saying a command and giving him the treat, pet him, and praise him. He deserves as much, he's learning how to shake hands after all. Repeat this until he will do it on command without a treat.