SEO is an extremely competitive field, especially if you want to work for yourself. Even if you excel in optimizing your own website, there is only one front page of Google. You can focus on local SEO and optimize your site for your community, however, if you live in a large city, you will still have a great deal of competition.
Firms that build websites and offer digital marketing services perform SEO for their clients as well, since the services go together. If you are only offering SEO, you may want to offer your services on Elance or Fiverr to get started, although you will have plenty of competition on these websites for freelancers.
You'll need testimonials from previous clients in order to get work, so you'll have to perform some free SEO services, contingent upon the client writing an honest review. Look for a deserving nonprofit or two in your community and a friend or relative's online business and offer your free services. Write an in-depth case study for each the sites that you optimize to place on your website, along with the review.
Show your expertise by answering questions on forums. You can include a link to your website, but don't spam the forums. Offer honest, helpful answers. You can also highlight your expertise by writing guest posts for blogs in the SEO and digital marketing industries.
If you live in a medium size community where there are enough businesses that have websites, ask you local newspaper if you can write an article about local SEO. If they are not interested, take out an ad that indicates that you perform local SEO services. Make your business appear exclusive by mentioning that you will only work for one company in each sector, so your clients are not competing with each other. It's a good alternative to PPC advertising, which is expensive in competitive industries.
You can also add a blog to your website talking about the latest SEO strategies and Google algorithm updates. Blogging is a great way to add fresh, relevant content to your website; everyone in the SEO field knows that content is king.