If you need a PGP public/private key, you can use this tool here but I recommend reading about PGP beforehand.
Instructions: enter your passphrase if you have one, then your private key, then the message, then hit "Sign". The signed message will be output below.
Disclaimer: No warranty is provided as to the quality of this signature tool. Full disclaimer can be found via LGPL. Use at your own risk. Don't be dumb about security. Messages are only as secure as you are.