From NY to Shanghai

I have booked a one way ticket to Pu Dong for the end of the month to visit a friend for some vacation time. I could not find a good deal on a roundtrip ticket but I am able to pay practically nothing for this flight by using some of my miles. I am a US citizen with a valid passport. However I am not sure how long I will stay exactly. I know it will definitely be less than 30 days. But not sure how short my visit will be. It really depends on when I can find a deal on a flight. Do I need a visa before leaving? Or can I apply for one once I get there and decide how long I will stay? Thank you!