Near the top of every page at CAF is a menu bar which says:
Home * Search * Groups * Members * Profile * Help * Log Out
To add or change a signature, use the Profile menu
Profile > Control Panel > Edit Signature
Then enter your signature and click the "Save Signature" button at the bottom of the page.
Oops... as a Trial Member, you do not yet have access to the signature privilege
That will change when you reach the status of New Member. This will happen automatically once you have been a member of CAF for a few weeks (three, maybe?) and have made a certain number of posts (15, maybe?)