You are required to have China visa to travel Tibet and it can be obtained from Chinese Embassy in Nepal through tour operator like Outfitter Nepal and a part from Visa you need to have travel permit to travel Tibet and the Visa and travel permit need to have beforehand to start the tour.

One need to confirm the tour itinerary/program through the operator before applying for the visa and permit as individual and independent travelers are not allow to travel Tibet and it takes three business days with valid passport to obtain visa and travel permit from Nepal.