need help

I am in Canada, I worked in a Canadian fastfood restaurant part-time started 2004 without a work permit. I am currently hold a study permit. The manager of the restaurant has never asked my SIN, so I did not provide it. My income earned were much lower than the minimum income in the province.

When it was the time to file the tax, I did not file that part, because I was not sure how should I do that. Recently, i received a "Confirmation of Employment" letter from the Revenue Agency asking about the 2004 income. Can anyone help me on how should I reply it?

Any help would be appreciated. thank you!