I am assuming you are using msys git. If you are, the editor that is popping up to write your commit message is vim. Vim is not friendly at first. You may prefer to switch to a different editor. If you want to use a different editor, look at this answer: How do I use Notepad++ (or other) with msysgit?
If you want to use vim, type i
to type in your message. When happy hit ESC. Then type :wq
, and git will then be happy.
Or just type git commit -m "your message here"
to skip the editor altogether.