All travelers (except for Chinese and Hongkong nationals) need a Tibet Tourism Bureau (TTB) permit to travel Tibet. My local partner will be helping you obtain a TTB. The fee is included in the tour price. Without a TTB permit, you won’t even be able to board a flight or train to Tibet.

Also, all travelers (besides citizens of Singapore, Brunei and Japan) require a valid Chinese visa in addition to a Tibet permit. To apply, head to your nearest Chinese embassy. Make sure to get your Chinese visa at least a month before your trip, as your tour operator will need it to get the TTB permit. 

Refer here for more details on how to travel Tibet.