Working the pecs
 

What is the best way to work the underside of my pecs?

Bench press variations and dips are about all you need. Get strong on these and you'll grow.

Great advice ; nuff said - Just tossing this in for conversation's sake.

If you are a new lifter, you do not need to focus on "shaping" a muscle. Your focus needs to be on getting strionger and learning the correct way to lift. I forget who said ( Arnold ?) but a wise man once said train for srength and size will come".
Althought some exercises place greater stress on certain areas ( such as decline presses for lower pecs or inclines for upper pecs) it is not possible to sub isolate a major muscle group. If you do as Btb suggested and work a variety of basic presses and dips into your routine your chest will grow.

Simple coumpound lifts with a focus on progeression will get you there. don't complicate it.

Wide-Grip Benches hit the outer chest really good.

Incline/Flat DB/bb presses, Db flys, dips. all you need.