1. For the A320 you need only git. For sourceforge there can be svn or git be used... I don't exactly get how it works my self... I just use git for it too...
2. On github, in the A320 repo, click on the top on "Fork" this will creat your own github A320 project where you can push your changes to. The A320 on sourceforge is in the hangar once we've got an updated stable version, we upload it there... you don't have to touch that at all.
The stable repo is on github, there's no development on sourceforge.
3. If it's like this (specific to the A320, ie. where the repos are) then here is okay, for general questions, better open a new thread.