I got the same itch back in 2005. Didn't know a thing about modern bikes, couldn't remember what I liked about cycling, but I wanted to give it a go. What worked for ME was buying a new entry level mountain bike, a Trek 3900, which was very inexpensive (maybe $400?). Rode that for quite a while, more than a year, and then began branching out into different kinds of bikes. I liked that what I bought was new, so it didn't need repairs (which I didn't know how to do anyway).

As it turns out, I'm not a mountain bike fan, and I've had two more since that one. But I had a helluva lot of fun on that 3900, and learned what I really wanted to do, and the rest is history. Only other tip I would add is that you'll want to make sure your bike is white. Just sayin'