1) Improving documentation
2) Bug reporting
These are easier to begin on. Just play around with Code::Blocks, and when you find a trick that makes it easier to use, write/expand pages in the wiki. If a feature starts acting strangely, report it on the bug tracker, or discuss it here in the forums.
3) Bug fixing
3) New features implementation etc.
My recommendation is to first start with something easy. Before you can do either of these, you will need to
build the trunk from source (it may take a little time at first, but once you understand the process, it is quite easy).
After that, start with reading the source of a small plugin (one that you already use regularly), and experiment with modifications on it.
Basically, start with working on whatever it is that you have the most motivation to work on.