If you receive a T4A with box 20 or 48 amounts, you are indeed self-employed for tax purposes as both of these boxes are used exclusively to report self-employment income. You're either a contractor or you've earned income from commission-based activities. You will enter your expenses under business expenses. Click on "Find" on the top right of your return and type in business expenses and click on Go- there you will find what you can claim. Click here for more information.
