Priorities... where do I start?

This is a great topic – in fact, I really would like some input on this. I’ve read a couple of leadership books, but I haven’t found much on how to prioritize. Naturally, as a leader we are supposed to make decisions made in the best interest of the group or the team. Is prioritizing tasks the same deal?

When working in corporate settings, usually the manager helps us prioritize projects based on decisions made by “upper management” and/or the board directors, etc. But when you are completely on your own, or asked to make those types of decisions– how do you know what to do first?