If the baby is born in Canada, he/she will automatically be Canadian. However, the US citizenship is not automatic. If you meet the basic requirement set forth by frege, you (or your child) will have until your child turns 18 year old to get US citizen for him/her. You can either get a consular report of birth abroad or a US passport for the baby (or both -- the consular report is like a birth certificate and doesn't expire, unlike the passport).
Google your local US consulate (Toronto, Montreal, Vancouver, etc.) for information on getting a consular report (you usually need an appointment, applications filled out, etc.) and the State Department for forms. Alternatively, you can apply directly for a US passport the next time you're in the US.
I have two Canadian-born kids and plan on getting consular reports of birth abroad for them at some point (it's ridiculously difficult getting appointments at my local US consulate). So can't be more specific about the process.
BTW, I'm sure you're already aware since you're a US citizen, but US tax obligations come with US citizen (i.e., the US is one of only a few countries with worldwide taxation). Not too much of an issue in Canada, which has a higher tax rate, but if your kid decides to say move to Hong Kong in future, it may be a concern.