It's difficult to get entry into Tibet. First of all, as the country is occupied by China you have to apply for a visa to China. You need to contact your Chinese Embassy to do this (If you put the words 'Chinese Embassy' plus the name of your country in a search engine, you will get a link to the relevant Embassy).

If you get a Chinese visa, you then need a Tibet Entry Permit. This is normally only available through a recognised travel agent from an approved list; you can contact a local travel agent for information or find further details here. You need copies of your passport, China visa and all other documents.

If you do travel to Tibet, you will find that visitors are restricted in the places they can go to. You may be able to apply for an Alien Travel Permit which
will give you more access; but you will need to ask about this through the
agency that supplies your entry permit.