If you want to contribute code you can get started by looking for issues marked as up for grabs. We also have the easy tag for issues suitable if you are unfamiliar with roslyn.
Also see the contributing guide.
You can also help by filing issues, participating in discussions and doing code review.
To start working on an issue, simply add a comment to the issue indicating you are working on implementing it.
Visual Studio 2015 is required for building this repository. The Visual Studio 2015 SDK is required for building the VSIX extension project and for debugging in an experimental visual studio hive.