Foreigner travelers are not allowed to travel on your own to Tibet, and Tibet permit is required for travelling in Tibet.
You could book a tour from a travel agency and travel with the travel agency to Tibet, and the travel agency will help you apply for Tibet permit.
If you are interested in Tibet, we would love to offer you Tibet tour packages. Here is a recommendation to you:
8 Days Tibet Everest Base Camp Tour.
Any further request or question, please feel free to contact with us.