Wish I had something to add that wasnt just parroting other guys --- but theyre right: decline bench press is the best single exercise for this. also can try cable flyes, but instead of seting the pulleys high (which is standard) set them low, so you're "scooping up" the stack. This is an underutilized move.

But I also second the WifeofBath: a good trainer will really help you if you are a novice. I see a lot of beginners making mistakes, which at best will stymie progress, and at worst can lead to injury.