Hi,
when you do a P and a B, your mouth does exactly the same things (= it moves the same way). The difference is that B is the "voiced" version of P. This means that when you say "B", your throat should "vibrate", and when you say "P" your throat should not.
It's the same difference that there is between F and V. When you say V your throat vibrates, when you say F it does not, but your mouth and tongue are in the same position. Try to say Vvvvvvv.... then Ffffffff.... Now you can try with P's and B's.
I'm sorry I can't help you more, but it depends on your native language. If there are no B's or P's in your language, then it's normal you're having a hard time pronouncing them. In this case, either you just try to imitate native speakers' sounds, or you can also try to see if this is any useful to you:
[link]Click on "Launch English Library". That's for American English, by the way. It'll show you how to move your mouth.