Which institution in China can make invitation for visit a friend there?

Hello,
I planning to visit my girlfriend for one month. She needs to make a invitation and send to me to apply for Chinese visa. Where she can make the invitation and/or ask for information? She is living in Shanghai.

Visit type is a standard 3 month one or I ll get one only for that period i ll stay?

Thank you in advance !
Regards
Alex Calistru, Romania