Fill in the data as follows:
Repository to clone: https://github.com/TrinityCore/TrinityCore.git
Destination: C:\TrinitySource
Subdirectory to create: <none> (delete anything in this field)
Branch: 4.3.4 (The Master branch is currently the 3.3.5a Wrath of the Lich King server, so be sure to select 4.3.4 for Cata)
Personal Repository: Yes
Click Clone. Within a few minutes all of the TrinityCore source files will be pulled into the folder.