I usually try to always change this when you sign-up, but in case I didn't , here's how to attach your signature.
1. Go to your profile
2. On the top Right hand it will list | Preferences | Signature | Avatar | Friends and foes | Topic is being watched
3. Click on "Signature" and add your signature.
4. Then click on "Preferences" and click the "Always attach my Signature" button then it will show on new posts.
5. Your done.