The first thing you want to establish is how long you are going for, the next is where you want to go.
Sit down with a map and make a list of the places, both cities and countries that you are interested in seeing. Map out a rough rough depending on your plane ticket - in a circle if you have to fly out of the same city you are flying into or in one direction if you are flying out of another place.
Now, have realistic expectations. Visit the "Fares and Schedules" section here: www.qksrv.net/click-711188-10358541 to determine how long your train rides will be. You want to be in a city at least 2 days without having to hop back on the train again.
I want to stress - you want to see these cities - and not just from the window of the train. Pack less, rather than more into your trip so that you can really enjoy it.
Make a rough intinary of where you are going and how many days in each place. Be sure to factor in travel time from one place to another.
Now, you can start looking up hostels you want to stay at and looking into making reservations if needed. See here to make reservations:
Definately buy a guidebook, it will help you with everything from accommodation to health matters.
Which one? That depends on you. Buy the most specific book you can, and by that I mean if you are just going to London, buy the London book rather than the England book. The more specific the book, the more information you can use.
The main ones are Lonely Planet, Let's Go, Rough Guides and Rick Steves books. Choose the one that you feel is most suited to you.