How to visit Tibet: non-Chinese visitors need to be part of an organized tour
As of 2018, the only people who can visit Tibet independently and without permit are citizens of China, Hong Kong and Macau
If you don't possess a passport from one of these three jurisdictions, unfortunately the only avenue for you to visit Tibet at the moment is to be part of a tour, whose organizers will apply for the Tibet Travel Permit for you
Tibet Travel Permit is name of the document that grants foreign tourists the ability to visit Tibet
It's a think piece of paper you'll be required to have on you at all times while in Tibet. Without this piece of paper, you won't be able to book any transportation to Tibet or any hotels in Tibet. Also there's a lot of security presence in Tibet so if you're stopped without this paper you could get in trouble
At the moment you cannot apply for Tibet Travel Permit on your own, only your tour organizers can do so
There are many tour operators for Tibet, ranging from group tours to private tours. I suggest searching on Google something like "Tibet tours" or use Tripadvisor
CAVEAT: what I said above is only applicable to the Tibet Autonomous Region, which is a province in China; the historical Tibet area is actually bigger that, so thealternative way to see Tibet independently without a tour or a permit is to go to the Qinghai province and western parts of Sichuan province. Qinghai province is the historical Amdo region of Tibet, and the western part of Sichuan province is part of the historical Kham region or Tibet. Foreigners do not need any permit or be part of a tour to visit Qinghai or Sichuan provinces
To visit Qinghai, you can fly to Xining, which is the capital of Qinghai. From there you can make use of long distance buses to get around
To visit the Tibetan part of Sichuan, fly to Chengdu first. It's the capital of Sichuan province and a very large city with a lot of transportation options to the western, Tibetan part of the province (like Kangding)
The main difference between Qinghai and Sichuan is the scenery. Western part of Sichuan is mountainous, whereas Qinghai is a plateau and grassy