Get your hands on everything related to social media and digital marketing. I truly believe this is the best place to start. The more knowledgable you are on each social channel, the easier it will be to manage the community.
From there, collaborate with your colleagues and put together a content calendar. Keep everything on topic with trends and current events, but make sure it relates to your brand.
Start posting and engaging with your audience. Remember that it's a two-way conversation. Find opportunities to add to your audience with Twitter searches and circles on Google+.
Keep track of your biggest supporters and fans with big followings themselves. Makes these types of people feel loved. Make everyone feel loved, really, but give these two types a little extra.