you have to build up your reputation in free lancing sites
you will have to rate a small money to projects at first to build up a good record
there are a list of free lancing websites today search them in google and get started
You my friend are unfortunately stuck in the chicken or the egg conundrum that many people get stuck in when they are first starting out. You need to do work to get and show your experience, you need experience to get work.
Freelancer.com, odesk, and a few others are great options as mentioned above at getting started. Unfortunately even the good paying jobs on there are going to require that you have a portfolio of work to show off. You can do your best to try and get jobs on these and in may require quite a few attempts but eventually you may make headway.
Inbetween trying to get those jobs you can also search online classifieds like ebay, kijiji, etc. for people looking for SEO services.
If none of that is working and you need to build a portfolio, I would create a few blogs/websites yourself. Get those to the top of search engine results, carry an excellent alexa rank - this will help to show potential clients that you know what you are doing with SEO because you can actually show your results. This will also make it easier to score freelance work.