You'll have to wait for 90 days (from the date you registered) before you can have a siganture.
The rules for signatures are below and can also be found at: 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.
Do not attempt to circumvent the 90 day rule by inserting a fake signature in your posts.
Signatures are intended to help you to promote your site, business, or other ventures that relate to you personally. For that reason, you can't let others use your signature, or rent your signature out to another party.
Create your personal signature in line with these requirements:
- Your signature's size is limited to 4 lines of 65 characters each. BBcode isn't included in the total number of characters in a signature.
- You can include links that lead to your site(s) or a product page.
- You can include a slogan (for instance, "UK's Best Hosting starting at £19.99").
- You can't include affiliate or referral IDs. (Links to Outwar and similar types of schemes are acceptable, as these are links to games.
- It should be obvious from the outset where any link leads to.)
- You can't use short URL services (such as TinyURL) in signatures to get around the points above
- You can't promote scams or make unenforceable offers (for example, "Earn $10,000 a month selling suntan lotion to dolphins").
- Pre-approved BBcode can be used, but must adhere to the following restrictions:
- Center and right-align tags can't be used.
- Code, Quote, PHP, HTML, Edit, Off Topic, Spoiler, Highlight, and HR tags can't be used.
- The largest font size permitted is size 3.
- Your signature can't contain images, but it can include smilies.
- Signatures can't be bought or sold, nor can they contain offers to buy or sell other signatures. This guideline includes linking to web sites that solicit the selling of signatures.
- Any attempt to include a 'hidden' signature will be removed and the user may receive an infraction.
Note that SitePoint is a no-follow forum. You will not receive a backlink from your SitePoint Forums signature.