If you are considered an employee, then you would need a T2200 - Declaration of Conditions of Employment from your employer to be able to claim employment expenses.
If you are considered self-employed (freelancer/independent contractor), then you'd have to fill out the self-employment form to claim your expenses.
Either way, you would need the receipts for any expenses that you weren't reimbursed for, as well as a log book that records your trips.
This CRA web page can help you determine if you are self-employed or and employee: Employee or Self-employed