When you hit 15 posts, wait about an hour. If you click on "Settings" at the top of the page, it gives you options to set avatar picture, etc. In the list on the left side of the screen, it will (eventually) give you the option to change signature. That's where you go to do it. If you're looking for one of the ones that shows how long you've been cigarette-free, just click on one of the ones that someone else has in their signature, answer the questions and click "create". Once it gives you the 2 options of how to copy it, choose the top one and paste it into your signature field, in your settings.
Hope this helps!