You should go to the Chinese embassy in Islamabad to apply for China visa because Lahore, Punjab Province is under its consular juridiction. Pakistani nationals are exempted from visa fees. However, For tourism, Pakistani citizens should first contact a qualified travel agency of China and settled down all the details of the tour. And then the tourist agency may arrange a visa notice form for a travel group (more than 5 people) and a group-visa for you. At present, individual tourists are not accepted.