First, check out the RoR site. If you haven't done ASP.NET MVC, or are unfamiliar with MVC, it's going to be a big difference. Rails is a BIG MVC framework, so it may do a lot of "magic" stuff that you are unsure about (but so does .NET).
I'd suggest reading about MVC first if you are unfamiliar about it, then go from there. It might be worth it checking out another lightweight framework first, as some people have issues jumping right into a big MVC framework.
Good luck! I've found Ruby to be a lot of fun.