I wouldn't recommend hiring anyone to do SEO for you. It's something that you can do on your own and that way, you'll have total control and understanding of what's happening on your site (to make sure no bad tricks are used).
Here are some good threads here at the forum that will help:
How Important is SEO? 5 Best Practice Tips for Successful T-Shirt Sites SEO Tips from forum member JDR8271 New Article on SEO
A good place to start is
Google's Webmaster Guidelines,
Technical Guidelines to Improve your page and
Quality Guidelines.
If you still have questions after that, feel free to post. We have some folks here in the forum that are happy to lend some tips.