Can I claim expenses like mileage or items purchased with a contractor job where I received a T5018?
Yes. You should be filing as Self-Employed reporting your income and expenses on the T2125 - Statement of Business Activities. When you file this way, you are still contributing to CPP, with the remittance being submitted with your tax return.
You can claim items like supplies and materials purchased, and instead of claiming "mileage", you will be claiming "Motor Vehicle Expenses". This means you should have kept receipts for your expenses like fuel and repairs, and kept track of the actual kms you used for business purposes and the total kms driven.
A complete list of eligible business expenses can be found in this link to the CRA information: https://bit.ly/2EDc2vB