Hi,
Social Media Marketing consists of creating your presence on social media platforms such as Facebook, Twitter, Linkedin, Google+, Pinterest...... utlimately the goal is to gain followers on these platforms and share useful information in your niche and from time to time market one of your services or products.
Take Twitter for example, what I like to do here is to create my page with a nice logo and banner; follow members that are already following my competitors (you can expect many of them to follow you back) in order to build a strong base of followers.
When you have your 100-200 first followers, you can start by sharing useful infos about your niche, by useful infos I mean provide solutions to the problems that your followers may encounter. When you feel that you have established enough trust with your followers you can begin promoting your products or services, and this is the whole idea behind social media marketing.
All the best!