How do I declare Foreign Income from a business I owned in Canada? I also paid Canadian taxes on revenues earned from this business.
Yes, as a US citizen you must report worldwide income.
To enter this income, open your return and search for foreign income. Click the "Jump to" link in the search results, answer Yes to Did you Make any Money Outside the U.S.? and follow the on-screen instructions. See screenshot below - click to enlarge.
Follow the prompts to enter your non-US income. TurboTax will also ask about the Foreign Income Exclusion, which may apply if you worked outside the US to earn this income.
Also, since you paid non-US taxes, you may qualify for the Foreign Tax Credit. See the instructions below on how to determine your credit/deduction: