I'm bad at PvP in minecraft?

Stay mobile. You never want to just stand still and hack away at someone, use all the buttons you have. Sprint, jump, move to the side you name it.
Try and use the terrain against them. If you see a hole near them that they haven't seen, keep pressure on them and try to force them into the hole. The same goes for mobs, just a tap with your sword can send them right into the path of a well placed creeper!

Try not to get into fights where it is 1+ vs 1, unless you are in that 1+ group. Avoid making enemies easily (people you kill as a newly spawned person may come back to haunt you when they have full diamond gear) and make sure you join a faction or group you know will have your back.

Get better at aiming with a bow, and know when to block with your sword so you don't take their critical hits full on with a sword. You can critical hit while; sprinting, jumping, and falling with a sword, so if they come running at you either move out of the way, place a block in their way to stop them or block with your sword.

You'll get better at it, don't worry! Just take your time and you'll have everyone on their knees in no time.