I used to have a similar problem with loose skin on my girls, they've tightened up a little bit now.
However, exercises for pecs include: Dumbbell flyes (lie on a bench and bring your arms straight out in the crusifix position, then keeping your arms straight, bring the dumbbells together above your chest) Bench press, either with dumbbells or a barbell, hold dbs at shoulder level, then push up to straighten arms.
You might want to try
www.exrx.com for muscle-specific exercises and little videos.