git is the Decentralized Version Control System (DVCS) used by the Preesm development team to version the PREESM and Graphiti sources. In order to get the sources of these projects and to contribute to them, you will need to install git and to create a Github account.
If you’re not familiar with git and plan to contribute to PREESM or Graphiti projects, you should first learn a little bit about git.
The git book is available in several languages and will present almost everything you need to know about git (you should at least read Chapters 2 and 3).
--help option after the different git commands will also help you to understand their goals and the way to use it.
You can also find a few git tips & tricks more specific to PREESM development on this page.
Finally, the Internet is full of resources about git (see for example StackOverflow questions related to git: https://stackoverflow.com/questions/tagged/git).
Install git & git-gui
Download & launch the installer: http://msysgit.github.com/
apt-get install git apt-get install git-gui
Download & launch the installer: http://code.google.com/p/git-osx-installer