Hi! I want to visit my Chinese friend in China this summer, which type of visa do I need to apply?

Beside of the summary question, here I have few more questions: I looked for some information, found that I need an invitation letter from him/his relatives. He currently is in Europe, so is that possible that he writes the invitation letter, or it must be done by someone who are in China at this moment? And is it easy to apply? Before my applying, do I need to prepare anything extra apart from the required documents, which are addressed on the Chinese embassy website?

P.S. I'm planning to visit China for one month (maybe will go to Thailand in the between)

Thanks for answering my question!