If you’re completely new to git then you might want to learn a little about its concepts and workflows. The Atlassian Tutorials are concise and very well written. A bit longer, but very useful, is the Pro Git book - chapters 2 and 3 are a good bootstrapping resource. These, and many other, illustrative and interactive online tutorials on git can be found as well:
One very important concept in git is branching, and there’s a very nice tutorial on that in particular:
There are also very nice GUIs for managing git repositories available. These will not be covered in this tutorial (partly because it’s a good idea to learn the commands, but also because most of you are likely to be logging into lxplus) but you may want to have a look at them while your code is compiling. You some popular GUIs (gitkraken is a good cross platform example; sourcetree works very well on a Mac). They can make working with git much easier once you’re working locally.
ATLAS uses the CERN instance of GitLab as its social coding site. The current help pages are useful as a guide and a reference.
GitLab has quite an intuitive interface though, so just going to the main offline repository and clicking around files, branches or commits is quite instructive.