If you are coming to Shanghai to experience Shanghai Expo 2010, you should probably get your tickets first, although you could buy them once you get here too. Beyond mere organizational considerations, tickets are also cheaper before the official opening on May 1, 2010.

If you are outside China, you can buy tickets from the official vendors listed below:

Hong Kong: China Travel Service (H.K.) Ltd.
Macau: China Travel Service (Macau) Ltd.
New Sintra Tours Limited
Taiwan: China Travel Service
Japan: JTB Corp.
Australia: China Travel Service
Helen Wong's Tours
USA & Canada: Ticketmaster Inc.
Canada: Tian Bao Travel Ltd.
USA: Peregrine Travel, Inc.
France: Agence Chine Tourisme
Germany: CHIMA Fernost Service Jun Zuo e. K.
Becker Travel Worldwide GMBH
Switzerland & Liechtenstein: Tian-Tan Horizon S.A.
Singapore, Indonesia & Brunei: Commonwealth Travel Service Corporation Pte Ltd
Singapore: Singa China Travel Service Pte Ltd
Vietnam: China Travel Service (H.K.) Ltd.
Thailand: G.M.Tour &Travel Co.,Ltd
Thailand: IMPACT Exhibition Management Co., Ltd.
 

More about Shanghai Expo 2010 on Chinatravel.net:

Shanghai Expo 2010 tickets: How to buy tickets in Shanghai

Shanghai Expo 2010 tickets: Types of tickets

Shanghai Expo 2010 tickets: Pricelist