what kind of documents i need to submit and how long i can stay there

Marie, you can apply for a tourist L type to PRC and you can ask your friend to write an invitation letter to you. In this case, the documents you need to prepare include your passport and copy of its data page, the completed application form affixed with a recently-taken half-length color photo, the invitation letter from your friend and proof of your financial support.
Usually, you may be given a 30-day tourist L type which will allow you to stay for 30 days in PRC.