How to save files in github

Question

So I recently learned how to use github and intellij at school. I am in a team of 3 and we need to do a java project and post it on github.
So my question is: if 2 or more of us work at the same time, and we clone the code from github, how do we make sure that the last version will have everything we have wrote? I mean, if A saves first, then B needs to save, does that delete A's work? How to proceed?