You will need proof of income (pay stubs, an employer letter, the last two years' notices of assessment if self-employed), your status in Canada (permanent residence or work permit), the source of the down payment and a credit check. With a pre-approval your offer to purchase is stronger in the seller's eyes.
It is not a guarantee: final approval depends on the property (appraisal, condo documents, condition) and on your situation staying the same. Avoid new debt or a job change between pre-approval and closing.