I'm an international student about to graduate. I never had a work permit, no income, no SIN so never filed tax return before. Stupid enough, I just realized I can use my tuition credit, so can anyone please tell me can I have my credit for the past four years or I can only file return for the previous year? And if I can, and if you would tell me any special things to be careful of, that would be great. Alternately wondering since I'm graduating in one month or so, do I need to try and get a ITN or SIN quickly or can I just wait till I have my post-grad work permit?
Note that Tuition, Education, and Textbook tax credit amounts can only be used to reduce tax owing and can be carried forward indefinitely to future tax years. Since you have no SIN number, I'm guessing that you haven't had any taxable income in the years that you've been a student. If that's not the case, I'm guessing you have a SIN that starts with a 9.
In any case, you can file your return for previous years right now. Assuming you would not have had any tax owing, you should not have to pay any penalties. See the Tax packages for all years page at the CRA. You must have received a T2202A slip or similar from your school for each year you've been enrolled.
Most common tax packages (TurboTax, Ufile, etc) will also allow you to prepare prior year's tax returns, although you'll likely have to mail them in as NETFILE is only valid for the current tax year.
Let me know if you need me to update this answer with any further details.