There is a snapshot version of GIT for Macs on the FG for Mac download page:http://macflightgear.sourceforge.net/home/
Do I only need GIT and FG if I just want to fly the planes?
Personally, I have 3 versions of FG and use them all frequently. There is no reason why you can't put another version in a place like a folder on your desktop. This would leave your current version in your Applications folder, and both will be usable.
usually don't code when I'm using my computer, so I'm not good at it... do I have to be that when installing and using GIT & FG?
The download is self installing, so you don't need to do any coding, just follow the procedure.
When I've installed GIT & FG and want to install new planes and scenery, can I do it the same way as when I only had FG?
Most of the aircraft on the main download page will fly with the Git version. There is a way to get the latest development versions, but you have to remember the snapshot version was made by Tat back in August 2010, so some of the more up to date developments on aircraft will not be usable with the snapshot.
I hope that helps.