For flight, seems Airasia usally provide cheap flight from Malaysia to Hangzhou, you can take flight to Hangzhou and then take bus or bullet train to Shanghai.
For accomodation, the hotels in Shanghai are so hot now, better book one in advance, thus you don't need to worry about accomodation once you getting there.
Ticket can be bought at the Expo Site, and one more thing, you need to prepare for the situation that you'll need to queue for a long time to enter some pavilions. So, make yourself pretty healthy when being there.