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 that you should have ready are tuition slips, child-care receipts, charitable donation, 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, 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 defense against an audit is to file your tax return every year and on time. Why give the CRA a reason to examine your tax situation? Get it done!
If you file a GST/HST return, make sure the revenue on your business statement T2125 matches the revenue that is on the GST/HST return.