If you were born in Canada, you became a Canadian citizen at birth. Since 1977, becoming a citizen of another country (even one like the US that has a renunciation clause in its citizenship oath) is not sufficient to relinquish your Canadian citizenship. To do that, you have to fill in a Canadian form and return it to Citizenship and Immigration Canada.

Assuming you were naturalized as an American after 1977 and haven't formally relinquished your Canadian citizenship, you still are a Canadian and should have no trouble getting a Canadian passport. Full details on how to do so are available at the Passport Office's website.

If you were naturalized as an American before 1978, your situation is more complicated. You likely did relinquish your Canadian citizenship but may be eligible to apply to resume it. Check out Citizenship and Immigration's website for details.

As for the US, the official position is that they recognize dual citizenship exists but don't approve of it. Current policy is to tolerate Americans acquiring and using other citizenships but that may change in the future. However, since you acquired your Canadian citizenship by birth, you are unlikely to have any problems using both passports. The one ironclad rule is that you must always enter the US on your US passport.