Yes, you have to apply for a Tibet Permit firstly. It is suggested that you visit Tibet with a group, especially to these remote area. On the other hand, if you would like to travel by train, please book the tour via travel agent as early as possible, since the train ticket is required urgently. Well, you may refer to this website's tibet tours~

only need to find an agency to help u get the tibetan tour permit. i dont know whether HKnese is also treated as the way to foreigners, but u'd better hire a private guide.U may contact a agency in Chengdu to see how they can help u! im an oversea student studying Chengdu now. and last year my friend and i did overland and trekking in tibetan region. we just found an agency who prepare us logistic: SUV car, driver who is also the guide, boarding and lodging,and prepare some camping equipments. We two,plus the guide had wonderful time there. this kind of travel service is good, if u don't wanna travel in group and price is not too high. that guide is quite helpful and we have become friends, this august i plan to trek another route in tibet still with their help. may this help u, have fun,:)

plus, u can arrange ur itinerary by ur own. so dont worry they may intervene. if u don't satisfy with arrangement like boarding or lodging, u can do it by urself when coming to the right place.but i suggest u follow their arrangment like booking hotel or tickets for they can get u many discounts, and the arrangement is good according to my experience. if u like they can even arrange u spend days in local tibetans home(i had that experience:)
wish u good luck!:)