For chest exercises, a lot of people think of bench press and pushups. And those can be great, but they actually work your arm muscles a lot, too. (Pushups, if done right, work your core; holding your body in that plank position works those stabilizing muscles.)
I think it's important to also do pec flies. Personally, I like to hold a free weight in each hand and lie down on a weight bench. With free weights, you know you're working the whole muscle because you have to steady the weight.
But with chest flies and bench press especially, you have to be careful not to injure your lower back. I see people at the gym all the time trying to life some really heavy weight, and they do it by arching their lower back! *Not* good. Your lower back should be in its natural, relaxed position at all times. If you aren't too tall, you might even want to bend your knees and place your feet on the bench. That really stabilizes your back so that you almost *can't* arch it.