hello i want to buy an iphone 6 please guide me i m pakistani

There are now 5 supported carriers in Pakistan from whom you can purchase an iPhone. ONLY the carriers and their authorized agents are authorized to sell the iPhone in Pakistan. iPhones from any other source are unauthorized gray market devices that were probably imported from another country. The carriers provide all service and support from the iPhone in Pakistan. If you buy a gray market phone, you will not be able to get it serviced unless you physically take it back to the country where it originated.

You can find the list of supported carriers here: Wireless carrier support and features for iPhone in the Middle East - Apple Support