Well, first, we need to know what you already have.
Xbox?
Broadband internet?
From there, all you really absolutely need is a kit and an ethernet cable. Just shop around, there's a front page deal right now for Starter Kits.
i'd start with the 3 month subscription that way if your ISP shuts you off for nonpayment, then you want lose much. plus you might sign up and decide you don't like it.