If I plan a tour into Tibet, the central land of China and Katmandu, which kind of documents shall I obtain? How shall I arrange the tour?
In whatever way, the basic documents you need are the Tibet travel permit, and the visas for both China and Nepal.

If you enter Tibet from Katmandu, you need to get the Tibet travel permits first, and then, regardless whether you have the Chinese Visa or not, you need to acquire a paper visa or have your travel permits stamped in the Chinese Embassy in Katmandu.     If you enter Katmandu from China, you can obtain the Nepalese Visa in the Nepalese Embassy in Beijing or Lhasa, while the latter is easier. Be prepared to spend two or three days to arrange the documents. As to the procedures, Central China-Tibet-Katmandu is a better choice for you.