The first and most important thing you need to do is to apply for a China visa. to be allowed to come to China. Because all visitors traveling to mainland China whether for business or pleasure, require a visa.
You can apply for a tourist visa either through the Chinese Embassy or Consulates around. But it needs about a week to get a visa. Alternatively you can contact a visa agent like this one: http://www.touchlawfirm.com which helped me for several times if you are on a rush trip.
You have to get a China visa first and then book your flight to Beijing, China and then fly to Lhasa.