My first advice- be flexible. If you try something and it doesn't work, try something else. No one way is the right way or the best way. So far as particulars go, I would suggest that he sit to pee until he is pretty well potty trained UNLESS he feels really strongly about standing- the reason being that at first the whole thing is not easy for them, and simplifying it by having him sit (ie, he doesn't have to think about "sit to poop, stand to pee") may make it easier. Also, if you want to potty train it is a good idea to ditch the diapers all together during the day, but use them at night at first. And as a child care provider I HATE HATE HATE pull ups. I think that if they are emotionally ready to potty train, they tend to respond better to wearing training pants (ie, "big boy underwear").
So far as keeping him on the potty- if he doesn't stay on, he's not ready. If he doesn't sit there long enough to do whatever he's not emotionally or intellectually ready to potty train, so you need to wait a few more months.
When you first start you should put him on the potty OFTEN- right when he wakes up, every hour, after every meal, and before nap. From there you just go with their natural rhythm as you start to figure it out. They have really neat timers out there specifically for potty training, or just use a kitchen timer. After a few days he may even hear the timer and go all on his own.
Other tips- keep him in loose sweat pants so he can be mostly self sufficient. I don't like using rewards for this, but it's not awful to use them- the reason I don't like them, though, is because if you need to use rewards he's probably not ready. But get him excited about it so he will want to do well. And never never make a child feel bad because they have an accident- talk about it, but don't yell or admonish. Doing that could very well set you back.
Just because he is taking his diaper off and saying poopoo, though, he may not be ready. If he puts up any stuggle don't force the issue. You'll just draw it out and make it torture for everyone involved.
Also, books are great, and let him watch your husband and/or you go to the bathroom, if you don't already. Good children's books- "Everybody Poops" and "Toilet Training"
Most of all, good luck!