You can get in Tibet either by China or from Nepal. There is direct flight from capital of Nepal, Kathmandu to Lhasa or you can also go through overland from Nepal. You can also get to Tibet from other cities of China. You can either take transit in those cities or stop for a night.
Travel by Air:
Fly from Kathmandu to Lhasa by Tibet’s airlines mainly connects Lhasa and other major cities of China. Lhasa Gonga Airport is located in Gonga County with 100km to Lhasa city. There is public bus between the airport and Lhasa city.
Travel by Land:
The Friendship Highway starts from the capital of Nepal-Kathmandu, traverses Hangmu Friendship Bridge (SinoNepal Friendship Bridge) at Kodari to Neyalamu County in Tibet and crosses Shigatse– the second largest city of Tibet to the capital of Tibet-Lhasa. It covers a distance of more than 900 km. It is easy to have a view of Mt. Everest when take this road. The best season to visit Mt. Everest is from early March to late May or from early September to late August, however, the weather is always changeable in this area.