Hi inicko!
A good start is
http://supertuxkart.sourceforge.net/Get_involved.
How to obtain the code and data is described in
http://supertuxkart.sourceforge.net/Source_control.
Before starting to code, please read
http://supertuxkart.sourceforge.net/Coding_Style.
Stay regularly in contact to the main developers. Use the IRC chat:
http://supertuxkart.sourceforge.net/CommunityI recommend you to either try fixing a small bug
https://github.com/supertuxkart/stk-code/issues?state=open,
or to add a tiny feature. Maybe you want to add a highscore feature to the grandprix. (When winning a grand prix, the total time is not stored in a file and not displayed in the GUI, as it is when winning a race on a single track.)
If the project you select is too complicated for a beginner, you might get disappointed and you might add extra work for the main developers.