I will meet you in the restaurant. I will meet you at the restaurant. Both are correct but I would use the first sentence if I intended to get there early and be seated already. The restaurant is on Zhongshan Street. You can say I am in a bus but usually people will say they are on a bus. We usually don't specify sitting or standing when in a vehicle. I am in a car. We never say we are on a car unless we are standing on top of a car. You can say you are in a bar or at a bar.

I will meet you in the restaurant. --> Yes, this means you want the person to go inside the restaurant to meet up with you. I will meet you at the restaurant. --> Yes. The restaurant is at Zhongshan street.(maybe in or on?) --> Should be "on Zhongshan Street" I am standing in a bus.(on a bus?) --> Usually we'd say "I am riding on a bus." You could say "I am standing in the bus." I am sitting in a car (on a car, at a car?) --> Yes, this is right. NOT "on" or "at." I am in a bar.(at a bar?) --> Yes, this means you are inside the bar; it is emphasizing location. You can also say "I am at a bar."

I would say the restaurant is in Zhongshan St (British English)