There are many different ways of working on github depend on whether you are working on your own repository, someone else's repository (without write permission), someone else's repository (with write permission), and etc...
So, let's talk about third way, 'someone else's repository (with write permission)'
- You need to make a Fork of a project. I would recommend you guys reading these 2 documentations that talks about what Forking does and how to work with it. Forking a repository & What it is for Syncing with main repository