download and install this (someone sent me link on IRC yesterday):
after installing create new folder, right click Git Extensions -> Clone
Repository to clone: https://github.com/TrinityCore/TrinityCore.git
hit clone button and wait...
when you get the source you should already know what to do with it
if you want to update that source, enter your folder, right click
on TrinityCore subfolder -> Git Extensions -> Pull
... let there Remote: origin, just hit pull button, that's all