Resource wise, check out John Bartholomew's youtube vids, chess fundamentals and his original climbing the ratings ladder series. Excellent advice.
My basic advice: Essentially if you have undefended pieces they're always a target. If the opponents have more attackers/defenders on a piece or particular square they can nearly always trade beneficially. So count attackers/defenders. Generally speaking, if the majority of your pieces aren't developed in a co-ordinated manner, you're not going to be able to mount an attack effectively.
There's a hell of a lot more to it than just this, but these are things to consider. Check out John's videos. They're really helpful.
Today I got to play a game with the chess coach of my school's chess club. We didn't get to finish the game, but make no mistake, I was not going to win. He was able to neutralize any attack idea I had and was slowly and systematically gobbling up my pieces. His advice for me after the game was to defend my pieces before trying to attack. I'm sure this is good advice, but it's a little vague. How do I know when I'm ready to mount the attack? Anyone have any resources they can recommend on this topic? Thank you.