Start putting a feature request for your idea...
GitHub has some great documentation on how to fork a git repository. Once you've done that, make your changes and send us a pull request! Be sure to include a detailed description for your pull request, so that a community manager can understand what change you're making, why you're making it, how it should work now, and how you can test that it's working correctly.
If your pull request meets the requirements and it hasn't been rejected by a product owner, then it will be scheduled for code review by one or more core committers. This process sometimes takes awhile: most of the core committers on the project are members of the project, and they have to balance their time between code review and new development.
Once the core committers are satisfied that your pull request is ready to go, one of them will merge it for you. Your code will end up in the next release, which usually which happens every week. Congrats!