In the most basic sense, social media is a means for individuals and organizations to communicate with each other online. Usually when people say "social media" they're talking about tools such as Twitter, Facebook, LinkedIn, StumbleUpon, Pinterest, etc.
However, in a certain sense, forums such as the one you're reading right now are also social media tools.
The easiest way to get started is simply to get started. Pick a social media platform where there's a good chance your target audience already hangs out. Get an account, follow some people and spend some time listening. When you've got a good feel for how people interact on that platform, start easing your way into the conversation.
It's a lot like being at a cocktail party. You can
just go barreling up to everyone you see and start yelling at them about how great you are... but that's not likely to have such a great result for you. Same thing if you do nothing but sit quietly in the corner and never talk to anyone at the party (or, worse yet, never even enter the room to start with).
On the other hand, if you take some time to get to know a few people, are respectful of their time and the other conversations they might be engaged in, and invest the effort to come across as interesting and entertaining, you can attract a lot of positive attention.
If you search the web on things like "getting started with social media," "effective social media techniques" or "how to use social media for business" you will find literally thousands
of articles with tons of good advice on how to get started.