You can take two routes, Sichuan-Tibet line or Yunnan-Tibet line.
Yunnan-Tibet line: Shenzhen to Guangxi, then Yunnan, Yunnan-Tibet line national highway 214 to Lhasa. Mainly visit Dali, Lijiang, Shangri-La, Meili Snow Mountain and so on, and then enter Tibet.
Sichuan-Tibet line, Shenzhen to Chengdu, take the Sichuan-Tibet line national road 318, mainly visit Chengdu and Ya &;an; ; An, Kangding, Ganzi and other places, and then enter Tibet.
The two routes will meet in Mangkang County, Tibet, mainly because the former one is different. Personally, if you have enough time and budget, it&;s better to go to Chengdu.