If you want to see which files will be deleted you can use the -n option before you run the actual command:
git clean -n
Then when you are comfortable (because it will delete the files for real!) use the -f option:
git clean -f
To remove directories, run git clean -f -d or git clean -fd
To remove ignored files, run git clean -f -X or git clean -fX
To remove ignored and non-ignored files, run git clean -f -x or git clean -fx