The first step in contributing to any Open Source project on GitHub is forking so that you can create Pull Requests. If you're not familiar with the idea of forking a repository, GitHub has a super simple tutorial!
If you're ready to get started, fork Forem's repository!
Once you've got a fork on GitHub, you can clone your fork to run the project locally:
git clone https://github.com/<your-username>/forem.git
As time goes on, you may need to refresh your fork. If you have trouble with that, you can read about keeping a fork caught up on Forem!