How do you build a virtual community?

We are preparing our product launch and I am a bit of a newbie when it comes to building a virtual community. Do you know the problem? A few questions: - Which channels do you use? (Twitter, PH, ...) - How many channels? - How do you attract followers? - Are there cool tricks and methods? - How many followers should you have in the different channels at least? Or if you have different things to say, please tell me.