I am Iraqi citizen, but I was studying in the United States, how can I apply for visa to China?

What is your purpose to China? The requirements are different based on different types.
In general, you can use your valid passport, an application form with a recent taken photo, an invitation letter and your proof of legal status in USA to apply at your nearby consulate/embassy.

Hello,
thank you for replying my question, right now I am studying in the USA and I have some Chinese friends I want to visit them, that is my purpose to go to China. I wonder that I have to have invitation to get visa or I'm just applying for it.

If you want to live with your friend instead of staying in a hotel, you need to have an invitation letter from them to get it.

If you just want to stay in a hotel, you can prepare your hotel booking in China as well as your round trip tickets to China to apply on your own.