An additional way to do that (from the command line):
git config --global diff.tool tkdiff
git config --global merge.tool tkdiff
git config --global --add difftool.prompt false
The first two lines will set the difftool and mergetool to tkdiff
– change that according to your preferences. The third line disables the annoying prompt so whenever you hit git difftool
it will automatically launch the difftool.