[Easy E]

"I was in the small blind (2-4 game) with As9s. There were no raises all the way around and just about everyone was in. Everyone seemed weak so I called"
I think you probably should have raised here. While it would have made your flop decision tougher, if anyone bet out behind you on the flop (assuming you check), you have a large volume of people to make catching your flush draw worth the raise.... plus you might get a free card on the flop if you don't catch well.

"The flop was 10h9c5d. I was more than willing to fold at this point but it was quickly checked all the way around to me so I decided to bet (mistake?)."
You screwed up here in your description- you couldn't have been checked to, in the small blind, on the flop. If you were actually in late position (button, cutoff) then the pre-flop raise becomes almost mandatory AND so does the flop bet.
Repost this and clean up the situation, will make the advice more meaningful.

"A 5h showed up on the turn. There was a check then a bet. The bet caught me off gaurd but I called and the first guy folded."
A ten bets here, an overpair bets here, a full house might or might not bet here (looking to suck in flush draws). A flush draw may bet here as well, whether s/he has part of the flop or not.
Again, sounds like you were in last position, not first- or did YOU check and someone checked behind?

"The guy turned over a 5 and a Jack (can't remember suite but I don't think he was on a flush draw). My question is, should I have known he hit trips with the five? I figured that if he had a five in his had he would not have came as far as he did, but obviously I was wrong."

That's not as important as the question "what could he be betting here that I can beat?" = only an underpair or a nine-Queen kicker. At low-limits, you can assume you're beaten most of the time here and safely fold.