Hey there!
It sounds like you're looking for Form T2125 where you can declare expenses for your business. Once you're in that form, you should see a vehicle expenses section where you can enter your mileage and other associated expenses. You can find more information about that form here: https://turbotax.intuit.ca/tips/t2125-tax-form-declare-income-and-expenses-from-a-business-or-profes...
You can also find more info about how to deduct mileage here: https://turbotax.intuit.ca/tips/how-to-deduct-mileage-if-you-are-self-employed-6390
I hope this helps!
