I'm a student and want to visit China, how can I apply for tourist visa to China?

You can just apply for the visa from the China Embassy in Islamabad, Pakistan with the following documents:
(I) Original passport with validity more than six months and minimum 2 blank visa pages.
(II) Photocopy of the first 2 pages (personal information) of the passport and the recent Chinese visa page if there is any.
(II) One fully completed visa application form in Chinese or English, attached with one of the applicant's recent color photograph (30mm X 40mm).
(III) Certificate of the relationship between you and the person in China you want to visit (original and photocopy).
(IV) Invitation letter from your relative you want to visit.
(V) ID card copy or passport copy of your inviter.
(VI) Copy of Chinese visa page of your inviter's passport.