SEO (search engine optimisation) is not complicated or difficult but it can be demanding of your time. SEO simple means optimising the on page text for relevant keyword weight, frequency and prominence and keeping the subject relevant to the page subject. 

The idea is to have better SEO than competing websites and thus rank higher and more prominently. SEO provides a cheaper long term solution for increased qualified traffic and generates customer enquiries that ultimately convert to sales.

One of the features of my SEO is to try to identify your uniqueness and use SEO to promote it. In theory SEO is easy and more about a 'common sense' than a science.

There are thousand individuals and companies who are using the internet to try to part you from your money. These charlatans are not only PAYING to appear in the sponsored listings of search engines i.e. Google Adwords, but they are STILL offering 'black hat SEO' techniques. Why anyone would choose to use the services of these charlatans is beyond me, just make sure they get your money.

My SEO is totally self taught, over a very long time, based on a commonsense perspective, and ANYONE can learn to do it. I am willing to teach you to by sharing with you my 'Beginner DIY guide to SEO. 

Some useful questions to ask an SEO include:
Can you show me examples of your previous SEO work? 
Do you follow the Google Webmaster Guidelines? 
What kind of results do you expect to see, and in what timeframe? 
What's your experience in SEO industry? 
How long have you been in business?

What does Google say about SEO? Does Google recommend working with companies that offer to make my site Google-friendly? Yes they do, and they also make it quite clear what to look out for and how to choose a good one.
http://www.google.com/support/webmasters/bin/answer.py?answer=35291

As you probably know SEO can cover a lot of subjects i.e. page text, alt text, tags, page names, navigation, architecture, links etc. But did you know t's also very important to have a good sitemap? 
Google prefers a sitemap in xml and here is a website that generates one for you, which you can then download and add to your server.
http://www.xml-sitemaps.com