Travel from china to Tibet

Good day sir,
first I would like to know which city is nearest to tibet so that I may start my trip from that airport for tibet.I am from mauritius and my plane get access to airport shanghai,chengdu,beijing,Guangzhou.I would like to know from which airport Tibet is nearest as I am planning to go to tibet by train.Also september 2017 will be my visiting month,thus I will need advice from you what are the acess certificate required .Thanks.