After you form your group and choose your idea, plan your activities together. Create a backlog for your project.
You do not need to be rigid with your plan and you will certainly not think of everything you will need to do, but planning will increase your chances of successfully finishing your project in time!
If this is the first time you work with git, it may seem a little overwhelming. Don't worry, though. It is simpler than it may seem at first.
In any case, here are some tips to help get started:
Choose a branching model. This will define how you collaborate on your project. A popular branching model with 1 master branch and multiple feature branches should work well for you.
In the articles "Branching Models in a nutshell", by Fernando Villalba and "4 branching workflows for Git", by Patrick Porto, some branching models are briefly presented and discussed. Take a quick look before choosing what works best for your team!
Use issues. You should use issues on Github to plan the work you have to do and keep track of the general development. Issues are also handy for registering bugs you find in your application.
Use pull requests. When you finish working on a new feature for you application, create a pull request to merge your changes to the master branch. At this moment, you will be able to review each other's code.