I'm starting a home-based business and make very little. Do I have to worry about income tax?
TaxTips.ca's Federal Tax Rates page has basic information about income tax in Canada, and it states:
Canadian federal income tax is calculated based on taxable income, then non-refundable tax credits are deducted to determine the net amount payable. For 2009, every taxpayer can earn taxable income of $10,320 ($9,600 in 2008) before paying any federal tax. [...]
Of course there are also provincial taxes to consider, but generally each province has a basic personal exemption amount. TaxTips.ca's page for Ontario rates lists $8,881 as the basic personal exemption in Ontario, for 2009. Find other provinces here.
According to the government website, the answer appears to be no in terms of personal income. However you may want to anyway to start creating RRSP contribution room as well as possibly qualify for GST/HST credit.
If your business is registered you are going to be required to file a tax return for it (and if it is a sole proprietorship then you would be required to file a T1 regardless).
When all is said and done, it seems that it's probably better to file rather than not file; even if you pay no income tax at least you are sure you won't receive a nasty letter from Revenue Canada in the future :)