What's the goal of mathematics? Are we just trying to prove every theorem or find theories which lead to a lot of creativity or what?
I've already read G. H. Hardy Apology but I didn't get an answer from it.

The question you've asked is actually a philosophical question, so it requires a philosophical answer. If we look abstractly at what mathematicians do, they are providing "scientific explanations" of pieces of mathematics, so: 
The goal is to provide a satisfactory scientific explanation of mathematics. 
There is a large literature on Scientific Explanation in philosophy. There are three nice articles in the Stanford Encyclopedia of Philosophy to provide a start:
The first, on scientific explanation in mathematics in particular:
http://plato.stanford.edu/entries/mathematics-explanation/
The second, on the philosophy of mathematics generally, which gives a scholarly treatment of many of the concepts in prior posts on this question:
http://plato.stanford.edu/entries/philosophy-mathematics/
The third, on scientific explanation more generally:
http://plato.stanford.edu/entries/scientific-explanation/
