You will need a signed T2200 by your employer. You can either claim expenses against your commission income or against your salary income. These links explain the difference in both claims:
https://turbotax.intuit.ca/tips/tax-deductible-expenses-for-commission-employees-5365
https://turbotax.intuit.ca/tips/can-i-claim-employment-expenses-12005
I hope this was helpful
