http://mmgen5jonsjns7bapjeoavuhqq5e3q75y4g2yxuwqt6hj77itutwbryd.onion/project/mmgen/mmgen-node-tools/src/master/LICENSE
Of course, your program's commands might be different; for a GUI interface, you would use an "about box". You should also get your employer (if you work as a programmer) or school, if any, to sign a "copyright disclaimer" for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see <http://www.gnu.org/licenses/>.