Welcome to the forums and I hope you will continue to contribute to the many active discussions.
For questions like this your first stop should be the FAQ/Rules link at the top of the page.
As stated in the [signatures section of [URL="www.sitepoint.com/forums/faq.php?faq=youraccount"]Your account and profile](http://www.sitepoint.com/forums/faq.php?faq=youraccount#faq_signatures):
Once you have been a member of the forums for 90 days you are allowed to add a signature to your posts. New users may not use signatures.
Be certain you adhere to the other rules about the format and content of your signature.