How do I generate a payment ID

If you are on Unix (Linux or Mac OS X), you can enter this command line in the terminal: openssl rand -hex 32

You can also use this website: https://www.grc.com/passwords.htm

If you have Monero on an exchange, you can generate a new payment ID there, that you can also use elsewhere. Note that when sending to an exchange, you must use the displayed payment ID

You cannot generate a payment ID with simplewallet or mymonero.com