SEO helps to define your site and its products, by using keywords that define your products and services so that your site will be picked up by the Search Engines. There are many optimization techniques that help to accomplish this, and unless you are tech savvy yourself, is generally best left to a person or company that is highly skilled in this area. There are MANY companies that do this, just be sure to find one that fits your goals and budget.
Also, this is only ONE part of the ingredients in promoting your website successfully. Some other ways of promoting your website is through social media platforms, such as facebook, myspace, etc. Also, depending on what your product and service is, you may consider buying advertising space, keywords, etc...
In all of these cases, it is very important that you create a well thought out marketing plan, have a budget in mind to execute your plan, and set goals and milestones and measure your results against those so that you constantly know where you are and where you want to be in x amount of time.