Hi, Kali! You can certainly strengthen and "firm up" the pectoral (chest) muscles by performing push-ups, supine (face up) pec flyes with dumbbells and supine bench presses with either a barbell or dumbbells. It's important, however, to develop and maintain strength in all of the muscle groups, so while strengthening the pectorals, you also want to devote time to strengthening the upper back muscles as well.
Cathe has a wonderful selection of strengthening workouts ranging from shorter total-body routines to longer split-set routines that target all of the muscle groups. Also, there's a great "Form Pointer" section on the Cathe site that both shows and describes how to perform select exercises for all the muscle groups - including the pecs. Check it out!
One thing to keep in mind - many women are under the mistaken impression that by strengthening the pectorals they will be able to alter the shape of the breasts themselves, and this is not the case. The shape of the breasts themselves is determined by genetics and by the amount of fatty tissue within them far more than the strength and mass of the underlying musculature. Pec strength is VERY important, so good for you for focusing on it!
Hope this helps -
Annette