Even if STK wont be part of the GSoC, it's still worth taking a look at the code. The best way is propably having a look at supertuxkart.sourceforge.net/Get_involved. The definitely first you need to do is compiling STK from source and getting comfortable with the tools we use (git, github, cmake). Then you can choose whatever task you'ld like to work, but it is suggested to start creating a challenge or solving a beginner task:
https://github.com/supertuxkart/stk-cod ... ginners%22