I need to exclude the folder App_Data from my Github but i don\'t know how.
I have a application which saves many files like jpg files in the directory: Source\\MyProjec
It should do exactly what you want. It will exclude files/folders with that string from your local repository path (the path where .gitignore is stored).
Keep in mind, this is not your only option. I'd read the help document linked below for a better understanding to get exactly what you need accomplished.
See here for more info: GitHub Help - Ignoring Files