This exercise / workout mainly targets upper chest / upper pecs. Triceps are the secondary muscles which are affected. To do this exercise effectively you need a raise platform like a bench chair where you can keep your feet on. This is like a regular push up just that your feet is on the bench . There is also a variation of this exercise in which you use fists instead of palms. Keep you back straight by contracting your abs.

How to do Decline Push ups video demo
Video Demo 1