Click on your username in the top right corner of the page. This brings up an "Account Details" page.
On that page, the 2nd column from the left has a categories list and about 2/3rds of the way down that list, is "Signature".
You can even ignore all I wrote so far and as long as you're logged into the forum, this link should take you there:
That should bring up a box, and then you will need fingers and something to poke them against to create letters in that box, then click/tap/nudge/poke the word "save" under that box.
If any part of that doesn't work, you probably have scripting disabled in your browser, or the site is just temporarily down to some extent again.
Edit: Or you don't have 25 posts like allisolm mentioned.