First and foremost you have to realize that no weight training exercises can improve the SKILL of throwing a football. What weight trasining can do is strengthen the muscles involved, which in turn increases their ability to produce force. With this new strength, you must practice the ACTUAL skill of throwing a football. This means football throwing drills in which you practice quick, intermediate, long throws like you'd have to perform in a game. Compound exercises which stimulate the arm muscles in addition to the larger upperbody muscles (chest, back) will probably suffice. Bicep curls, tricep extensions etc are also fine as well as shoulder exercises. Strengthening the body as a whole is a better idea than trying to isolate and strengthen the specific muscles involved in throwing a football.