The easy place to start is by analyzing your current position in the social landscape - and chances are, someone's already talking about you online. Take a look at what's being said, and analyze the impact of those conversations.
You also need to understand your motivation for using social media. Rather than taking an ‘everyone is doing it' approach, you'll need to clearly define your goals and outline what you believe social media will help your organization achieve. All social tools are not created equal, and some work better than others depending on the desired outcomes and target audiences. There are numerous inexpensive tools that can help you create a launching pad for a social media presence that will, over time, be effective.