Bring your last two years of income documents, a list of your existing debts and an idea of your available down payment to a lender or mortgage broker, and ask for a written pre-approval rather than a verbal estimate, since only the written version reflects an actual underwriting check.
A CMHC minimum credit score of 600 applies to an insured mortgage, so check your credit report before applying rather than being surprised by it mid-process. Pre-approval is typically valid for a set window, often 90 to 120 days, so timing it close to when you plan to start looking matters.
A written pre-approval is the only number worth planning around, since everything else is an estimate until a lender actually checks your file. Get your hand-reviewed valuation from Joel Dyck.