By doing SEO of your own website you can get huge amount of visitor. You can sell your products to the visitor or get traffic from the visitor. By increasing traffic you can earn a lot of money using Google Adsense.
We can earn a lot doing freelance work on SEO. In freelance market place like Odesk, Elance, Freelancer etc, there are lots of work on SEO. We can apply for SEO jobs in those freelancing sites and bid there as well as can earn money by finishing the work.In internet there are a lot of tutorial on SEO. We can collect them and learn SEO. SEO job is not a difficult job. Anyone can do it by taking some necessary knowledge. You can get much tutorial about SEO from my site.