I'm trying out the danish gambit and for danish gambit accepted my opponents often play developing moves such as knight to C3 or knight to e3, or developing the bishop to b4.

For example, I am playing against the java program on chess.com with the danish gambit, and the opponent always play knight to C3 after danish gambit accepted, and I couldn't figure out what to do after that.

Also, what type of books and web sites can I learn from in order to improve on this style of play?

You are White, I'm assuming.

Although it's not the best video out there, YouTube - Chess Lesson: Danish Gambit Opening should get you started with some of the basic ideas.

I'm personally not familiar with the gambit, although I have been interested in it from time to time.

Also, there's a good example game on chessvideos.tv, which shows many tactical ideas that White has in the opening. katar - Sacrificial Danish gambit [9:30]

I think your notation is messed up, since it's quite difficult for black to get a knight to c3 or e3 in the Danish. Do you mean c6 and f6?

"Danish Dynamite" by Karsten Muller and Martin Voigt is great for the Danish ... and also the Goring (which you should consider checking out if you are interested in this type of play).

After the Danish Gambit Accepted (1.e4 e5 2.d4 exd4 3.c3 dxc3 4.Bc4 cxb2 5.Bxb2), you say you're confused with what to do against 5...Nc3? I'm assuming you mean 5...Nc6. In that case, you can just continue developing with Nf3.