If you want to work on the OpenMW code, here is a check list for you:
- Register on our Forum
- Register on GitHub
- Fork the OpenMW repository on GitHub
- Clone your fork locally
- Build OpenMW
- Make sure your build of OpenMW does run
- Register on our bug-tracker
- Read the Developer Reference and especially the Policies and Standards
If you managed to do all that, you are ready to start coding. Try to find a task on the roadmap  for the current release. If you find something on the roadmap not scheduled for the current release that might be okay too (some of these tasks have other unfinished tasks as prerequisite though).
Once you have found something, please make a thread in the developer section of the forum; ideally before you start coding, because we might be able to provide additional input.
If you can't find a suitable task on your own or are unsure about the suitability of a task, please ask for advice on the forum.
Occasionally we also have non-coding tasks. These mostly involve work on the wiki or research (figuring out game mechanics parameters by experimenting with Morrowind).