http://stackoverflow.com/questions/10564/how-can-i-set-up-an-editor-to-work-with-git-on-windows

On 32 bit OS:

$ git config --global core.editor "'C:/Program Files/Notepad++/notepad++.exe' -multiInst -notabbar -nosession -noPlugin"

On 64 bit OS:

$ git config --global core.editor "'C:/Program Files (x86)/Notepad++/notepad++.exe' -multiInst -notabbar -nosession -noPlugin"

Obviously the C:/Program Files/Notepad++/notepad++.exe part should be the path to the Notepad++ executable on your system. For example, on Windows 7 it's likely to be C:/Program Files (x86)/Notepad++/notepad++.exe

Git uses Vim to edit so if you need to save and exit press, ESC + Z + Z (capital Z's).