First of all, it's important to make sure that you can actually get onto the trains you need. If your planning on taking a long trip around Europe, a Eurail pass is probably the best thing to get. It serves as your ticket on any of the train systems that accept that kind of pass, which is most of Europe. You can get different kinds of Eurail passes, so just find the one that's best for the kind of trip you'll be taking. It's good to order it as soon as possible since it has to be mailed to you; you can't just print one out. I would say order it no later than a week before your trip, but even that may be cutting it close depending on how long it takes to mail it to you. Once you have it, you just go to the train station and they'll help you to get it activated. Here's a link to the website so you can find out more:
http://www.eurail.com/
However, a Eurail pass DOES NOT serve as a reservation. When we started on our Eurotrip the other week, we almost didn't even get on the train that goes from Sweden to Copenhagen because it required a reservation in order to get on, which we hadn't gotten at that point. We lucked out and were able to squeeze one in for ourselves at the last minute, but it's good to get that kind of thing sorted out as soon as possible. It's a good idea to go to the train station and ask them to reserve a seat for you at least one day in advance. When traveling, it's good to get the reservation for your next train out as you arrive at the station at your destination. So the Eurail pass basically saves you time and money on getting tickets, but you need to get a reservation separately for some trains, especially the busier systems like the ones going in and out of Paris for example. And while in the cities, you may need a different kind of pass for riding the local metro lines, so keep an eye out for that, too.
When navigating the train system, you can always ask for directions or specifics at the information desks at the train stations. (Most people speak good English over here, so don't be shy about talking to people.) Most of the time, trains are listed by what direction they're going in. So say you're going to Hamburg, your train may be listed as going to Berlin since it'll be going in that direction. Even though your specific destination may not be the one listed, the train will still stop there along the way. The individual stops are usually then listed on the display screens on the trains, or else on the time tables and schedules you can have printed out for you at the stations. You'll also need to stay alert and keep track of when you'll be arriving at your stop. They don't always announce over the PA when they make it to certain stops, especially on the night trains. (Don't want to wake anyone up.) So you need to know when to be ready to get off and/or transfer from train to train.
It's also good to book your hotels or hostels in advance before setting out on your trip. If you've read my other entry, you'll know that one night we didn't have a place booked for the one night we were in Berlin, and we ended up getting stranded on the streets at night. Not fun. So be sure to always have a place you can stay, and never assume that there'll be a place available, even in a large city with lots of hotels and hostels. One reason is because there's the chance they could be fully booked, or else are closed. (Yes, that did happen to us.) Unlike in the U.S. where hotels usually have someone sitting desk all 24 hours in the day, a lot of hotels and hostels in Europe are only open during certain hours in the day. So also make sure you'll arrive in time to get into the hostel you'll be staying in, otherwise notify them so that they'll know when to be expecting you to check in.
Hopefully these tips will prove helpful to anyone planning to travel around Europe. In the end, just relax, plan ahead, be prepared, and HAVE FUN!