Before going to Tibet, you should apply a China Visa. Here are the websites of the Chinese Embassies and Consulate-Generals in some countries. You can find the address of the Chinese embassy or consulate-general the nearest to you.
Chinese Embassies:
Chinese Embassy in Australia http://www.chinaembassy.org.au
http://au.china-embassy.org
Chinese Embassy in Austria http://www.chinaembassy.at
Chinese Embassy in Belgium http://www.chinaembassy-org.be
http://be.china-embassy.org
Chinese Embassy in Brazil http://www.embchina.org.br
Chinese Embassy in Bulgaria http://www.chinaembassy.bg
Chinese Embassy in Canada http://www.chinaembassycanada.org
Chinese Embassy in Czech http://www.chinaembassy.cz
Chinese Embassy in DPRK http://kp.china-embassy.org/eng/
Chinese Embassy in Denmark http://www.chinaembassy.dk
Chinese Embassy in Egypt http://www.chinaembassy.org.eg
http://eg.china-embassy.org
Chinese Embassy in Estonia http://www.chinaembassy.ee
Chinese Embassy in Ethiopia http://et.china-embassy.org/
Chinese Embassy in Finland http://www.chinaembassy-fi.org
Chinese Embassy in France http://web.amb-chine.fr
Chinese Embassy in Germany http://www.china-botschaft.de
Chinese Embassy in Hungary http://www.chinaembassy.hu
Chinese Embassy in Hellenic http://gr.china-embassy.org
http://gr.chineseembassy.org
Chinese Embassy in Iceland http://www.china-embassy.is
Chinese Embassy in India http://www.chinaembassy.org.in
Chinese Embassy in Indonesia http://www.chinaembassy-indonesia.or.id
Chinese Embassy in Ireland http://www.chinaembassy.ie
Chinese Embassy in Israel http://www.chinaembassy.org.il
Chinese Embassy in Japan http://www.china-embassy.or.jp
Chinese Embassy in Kenya http://ke.chinaembassy.org
http://ke.chineseembassy.org
Chinese Embassy in Lithuania http://www.chinaembassy.lt
Chinese Embassy in Nepal http://www.chinaembassy.org.np/
Chinese Embassy in Mexico http://www.embajadachina.org.mx
Chinese Embassy in New Zealand http://www.chinaembassy.org.nz
http://nz.china-embassy.org
Chinese Embassy in Norway http://www.chinese-embassy.no
Chinese Embassy in Papua New Guinea http://pg.china-embassy.org
Chinese Embassy in Peru http://www.embajadachina.org.pe
Chinese Embassy in Poland http://www.chinaembassy.org.pl
Chinese Embassy in Portugal http://www.embaixadachina.pt
Chinese Embassy in Romania http://www.chinaembassy.org.ro
Chinese Embassy in Russia http://www.chinaembassy.ru
http://ru.china-embassy.org
Chinese Embassy in Saudi Arabia http://www.chinaembassy.org.sa
Chinese Embassy in Singapore http://www.chinaembassy.org.sg
Chinese Embassy in South Africa http://www.chinese-embassy.org.za
Chinese Embassy in Spain http://www.embajadachina.es
Chinese Embassy in Sweden http://www.chinaembssy.se
Chinese Embassy in Switzerland http://www.china-embassy.ch
Chinese Embassy in Tajikistan http://www.chinaembassy-tj.org
Chinese Embassy in Thailand http://www.chinaembassy.or.th
Chinese Embassy in the Netherlands http://www.chinaembassy.nl
Chinese Embassy in the Philippines http://www.china-embassy.org.ph
Chinese Embassy in the Republic of Korea http://www.chinaemb.or.kr
Chinese Embassy in the United Kingdom http://www.chinese-embassy.org.uk
Chinese Embassy in the United States http://www.china-embassy.org
Chinese Embassy in Turkey http://www.chinaembassy.org.tr
Chinese Embassy in Uganda http://ug.china-embassy.org
http://ug.chineseembassy.org
Chinese Embassy in Uzbekistan http://www.chinaembassy.uz
Chinese Embassy in Zimbabwe http://www.chinaembassy.org.zw
Chinese Permanent Mission to http://www.chinesemission-vienna.at
the United Nations and other International
Organizations in Vienna
Chinese Permanent Mission to http://www.china-un.org
the United Nations in New York
Chinese Permanent Mission to the United http://www.china-un.ch
Nations Office at Geneva and other
International Organizations in Switzerland
Special Commissioner's Office of the http://www.fmcoprc.gov.hk
Chinese Foreign Ministry in Hong Kong SAR
Special Commissioner's Office of the Chinese http://www.fmcoprc.gov.mo
Foreign Ministry in Macao SAR
Chinese Consulate-Generals:
Chinese Consulate-general in Auckland http://www.chinaconsulate.org.nz
http://auckland.china-consulate.org
Chinese Consulate-general in Chicago http://www.chinaconsulatechicago.org
Chinese Consulate-general in Fukuoka http://www.chn-consulate-fukuoka.or.jp
Chinese Consulate-general in Houston http://www.chinahouston.org
Chinese Consulate-general in Khabarovsk http://www.chinaconsulate.khb.ru
Chinese Consulate-general in Los Angeles http://www.chinaconsulatela.org
Chinese Consulate-general in Melbourne http://www.chinaconsulatemel.org
http://melbourne.china-consulate.org/chn/
Chinese Consulate-general in New York http://www.nyconsulate.prchina.org
Chinese Consulate-general in Rio de Janeiro http://www.consulado-china-rj.org.br
Chinese Consulate-general in San Francisco http://www.chinaconsulatesf.org
Chinese Consulate-general in Sydney http://sydney.china-consulate.org
Chinese Consulate-general in Toronto http://toronto.china-consulate.org
Chinese Consulate-general in Vancouver http://www.chineseconsulatevancouver.org