Travellers wishing to travel to Tibet while on a visit to China need to first apply for a China visa UNLESS entering from Nepal where they need to apply directly at the Chinese Embassy in Kathmandu.
If you plan to visit Tibet you need to get a China visa first but you should NOT mention any Tibetan cities on your application. Once you receive the China visa you can then apply for a Tibet Permit to a registered Chinese travel agency to apply on your behalf.
Here is the material to apply Tibet Permit.
1. Copy of China Visa
2. Copy of Passport
Here is the official link for China Consulates.
http://www.fmprc.gov.cn/mfa_eng/wjb_663304/zwjg_665342/2498_665360/
Here is the official link for China Embassies.
http://www.fmprc.gov.cn/mfa_eng/wjb_663304/zwjg_665342/2490_665344/
