How to participate
There are several ways to participate in the Git for Windows project. First off, have a look at the governance model of the project.
A lot of documentation wants to be written, still. A good place is the Git for Windows wiki. Tutorials, step by step instructions and general background information, etc. Even just keep the links up to date and reporting inconsistencies is helpful.
Fix bugs or add features in the Git code itself
Also have a look at this page for tips and tricks revolving around building and debugging Git.
Please also sign off your work, as asked by the Git project
After that, submit a Pull Request.
Sometimes there are good suggestions how to improve the patches in a Pull Request. When that happens, just rewrite and force-push the branch.