how do you exploit weakness

sometimes it is hard to see a players weekness when you are playing

To exploit weakness, you must see it first. From what you're saying, it sounds like you're having trouble recognizing weakness in an opponent's position. I'd say study tactics to improve this skill. Here on chess.com, you can do the daily puzzles and practice with the tactics trainer. These are great places to start. There are also many books on tactics. I'm a fairly new player, as well, so I can't help you with book titles, but some other people in the forums here may be able to point you in the right direction.

If you can't find a tactical exploitation then you can always use the principle of 2 weaknesses. You keep threatening two seperate weaknesses to push back the opponent. e.g.