First you should have a Chinese visa:
Apply for Chinese VISA from local embassy of China in your country.
Find a travel company and book tours:
MY TIBET TOURS offer various Tibet tours and trekking packages. Please check Tibet Tours for details.
Make the tour company help you apply for Tibet travel permit:
If your occupation is not diplomat, journalist or government official, you won’t have difficulty applying the permits. You just have to offer your personal information, copies of your passport and Chinese VISA, your tour company will help you apply for it. The charge is usually included in the tour package.
Before taking the tour, make sure your health is allowed.