You should keep your supporting documents for six years.
If you owe tax, the penalty is 5% of the outstanding balance plus 1% a month until the tax return is filed, to a maximum of 12 months. This works out to a maximum of 17%. If you have been late in filing your return in any of the last three previous years, the penalty can be more.
You will need all your T-slips. For example, you will need your T4, T3, T5, T5008, and RRSP deduction slips etc. Additional slips you should have are tuition, child-care receipts, charitable donations, etc. If you use your home or vehicle for work and have a T2200, you compile your fuel receipts, insurance costs, maintenance costs, monthly lease costs, and purchase invoice if you purchased a new vehicle in the year.
Remember to bring the last tax return that you have completed and your notice of assessment if you can find it.
The best defence against an audit is filing your tax return on time annually. Why give the CRA a reason to examine your tax situation? Get it done!
If you file a GST/HST return, ensure the revenue on your business statement T2125 matches the revenue on the GST/HST return.
A.M.I. CPA Professional Corporation can discuss with you more details on your personal taxes.