You may either take a train or a plane.

We flew from Beijing to Xian, then from Xian to Lhasa, then from Lhasa to Chung Shen, then from Wuan to Shanghi. It's obviously cheaper on the train, and although you did not ask, I'm sure bus transportation is also available. All flight were very good (food and drink served on all flights), and the flight attendants are just like they used to be before 9/11. Security is tight, but not overly so. They have bullet trains from Beijing to Xian and Beijing to Shanghi; I saw one go by one time when we were on our tour bus and they move very fast. I would certainly choose air, but if you are on a tight budget, I'm sure a train is a good option. Be very careful when travelling in and out of Tibet as the rules are very strict, besides the Visa, you need a special travel permit (check with your travel agent). You will love China and fall in love with the Chinese people...great country, probably moving to fast economically, but great people who only want to make life better for them and their families (and what's wrong with that?).