I've decided I want to bicycle across America next year. What should I need to know bike-wise to be ready?

I missed out on a golden opportunity a couple years ago and have finally found a chance to go on a three-four month journey across America by bike (starting here in Washington). I know first and foremost I need to learn about bikes. I'm going to need and purchase a really good bike that can safely make this journey of ~3100 miles.

What are you recommendations? What should I look for in a bike and why?