Find an area you can help with and do it. Open source is about collaboration and open participation. Try to make your code look like what already exists and submit a pull request.
The [list of issues](https://github.com/mimblewimble/grin/issues) is a good place to start, especially the ones tagged as "help wanted" (but don't let that stop you from looking at others). If you're looking for additional ideas, the code includes `TODO` comments for minor to major improvements. Grep is your friend.