First of all, make up a key. You could pick a long nonsense phrase like:
The fribbits seem glinky today, don't you think?
or some gibberish pounding on the keyboard. (But watch out! That's often a lot less random that you might think. That's how the Russians used to make one-time pads for spies. The alternation of left and right hand keystrokes on mechanical typewriters was clearly evident from captured pads.)
But why go to the trouble when Speak Freely will make up a key for you on the spot? The Options/Create Key... menu item generates a seed number from the time, date, and a variety of information about your computer and uses it to initialise a highly-secure IDEA-based random number generator. (Commercial users: read the "Patent issues" section before using this feature.) The key is displayed in a dialogue box:
Each time you press the "New Key" button, another key is generated, containing 128 bits of information, the same as an IDEA key and more than twice the 56 bits of a DES key. The text of the key is automatically selected and may be copied to the clipboard with Ctrl+C and pasted into a key field in the Options/Connection... dialogue box and an electronic mail message to the person you want to speak with. The message might read something like:
Hi! I'd like to talk to you around 21:00 your time tonight with Speak Freely. I've generated a key of:
for this conversation. Please paste this key into the IDEA encryption box when you set up the connection to my machine. I'm looking forward to chatting with you!
Encrypt the message with PGP, specifying the public key of the recipient, and send it winging its way over the Internet.
When your friend receives the message and decrypts it using PGP and his secret key, he will know the key to use for your forthcoming conversation. You can either generate a new random key for each conversation (as PGP does) or, if you trust the other person (and yourself!) to keep the key secret, use it for multiple conversations with that individual.
This technique permits you to exchange keys for conversations with people you've never previously communicated with in any manner, as long as you trust their published PGP public key to actually be theirs. Of course when you let Speak Freely generate the key, you're trusting me to not have installed a "back door" that allows me to know what key you'll get, or to have accidentally introduced a bug which makes the keys predictable.
If RSA weren't patented, or the owners granted a blanket license to use it noncommercially, we wouldn't have to go through this rigmarole, but it is and they haven't so we do.