I want to go to Shanghai to Disneyland, staying only 2 days. How do I get a visa. I'm a US citizen

Claudia, actually, it’s possible for you to enjoy free entry to Shanghai. If you travel like this: US-Shanghai-HK-US, you can enjoy the free transit in Shanghai and in this way, no entry permit is needed.
Otherwise, you need to apply for a tourist L type for Shanghai. The required documents include your passport and copy of its data page, the application form, a passport-style photo, round trip air tickets and accommodation booking record. Next, submit the application to the embassy of PRC to apply for it.

Could I also get the free visa if I travel USA-Pvg-Nrt-USA ?

If your layover time in PVG is less than 144 hours, you can enjoy the free transit there.