一般不说“我要准备去上海看EXPO”,这就是说“I want to prepare to go shang hai...."。一般是说“我要去上海看Expo"。而"我准备去上海..."是"I plan to go Shang Hai"和“我正准备去上海..."是"I am preparing to do Shang Hai."

we usually say " 要去" or "准备去". both of them mean " going to" , "正" means" present progressive.

要准备 means you have the idea to EXPO,but maybe no real plan 正准备 means that you are preparing for EXPO,or we can say that you have got a plan

我要准备去上海看EXPO,一般将来时 "我正准备去上海看EXPO"是现在进行时ing

They are definitely different. The first means "I am about to go watch 2010 Shanghai Expo .or I will be packing/preparing to go watch 2010 Shanghai Expo."All this 2 meanings are not immediately going. The 2nd means "I am leaving for 2010 Shanghai Expo" You probably already on the road or you are driving to the airport,or you are packing to leave. Hope you can understand if that makes sense to you. =)