i have an iphone 4 locked to at&t and i want to use my phone for the telus network how will i accomplish this

iPhone: About unlocking - http://support.apple.com/kb/HT5014 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

iPhone: Wireless carrier support and features - http://support.apple.com/kb/HT1937 - including a list of which ones offer unlocking.

Some carriers will not accept unlocked phones - https://discussions.apple.com/thread/4530650?answerId=20359060022#20359060022 - "Virign Mobile, like all CDMA carriers, will only activate devices on their network that were initially sold for use on their network."

https://discussions.apple.com/thread/4535859 - topic detailing the procedure in unlocking an AT&T phone

Contact AT&T to determine if you qualify for their unlocking service.

Here are AT&T's official requirements and procedure for unlocking an iPhone:

https://www.att.com/deviceunlock/client/en_US/