Social Media and SEO are two different methods of promoting your website. Social Media refers to obtaining visitors from social media network (such as twitter, facebook etc), while SEO deals with gaining visibility directly through search engines. Nevertheless, in the past years the two processes became more and more bound together. The latest Google algorithm updates show clearly that the presence of a site on social media networks (especially Google+ and Facebook) can affect your ranking in search engine.
The best way of starting to understand Social Media is to open a facebook page for the website you are trying to promote, share it with your friends and then attract more and more visitors and fans, by sharing viral content.