Well, you will pay
taxes in both countries, because you're not exempt from payroll/social security withholding in the US on your US-source income, and presumably you've got income in Canada too, e.g. you get a T5 from the bank to report your bank interest.
But basically if you live in Canada, you file your
taxes like any other Canadian does, the only real difference is that you have to claim a foreign tax credit for any income tax you paid in the US (you report your total worldwide income on your T1, use the exchange rate on the days you were paid, work it out from
www.bankofcanada.ca). As Canadian tax rates are usually higher than US rates you will probably owe some income tax to the CRA.
In the US you file a 1040 with your Canadian address on it, because you're a US citizen unfortunately you also have to file for a foreign tax credit for any income tax you paid in Canada on Canadian-source income, e.g. the income tax you pay on your bank interest. You also report all your income on your 1040, but presumably there will be little tax to pay because your US withholding will cover it (in fact you may be due a refund).
This assumes your principal residence is in fact Canada.
It's hard to comment further because it's not clear on what your specific situation is, but if you have a look on the IRS website under "non-residents" it will give you some idea of what the situation is, plus read the instructions for the general guide for the T1.
The main thing to bear in mind that people come unstuck on in your situation is that you cannot be resident in both countries for tax purposes simultaneously - it's one or the other, and there's always some caveat in how they determine "residency" that if you have "closer ties" to another country that is your tax home regardless of whatever test it is they use.
The US also goes out of their way to make it more complicated because US citizens are taxed on the basis of their citizenship, rather than just where they live, so you have the appearance of filing US
taxes like people resident in the US (because you file a regular 1040 instead of a 1040NR), but in fact you are not in the same situation and slightly different rules apply.
If you need to adjust your return you need to fill in a 1040X in the US or a T1 adjustment in Canada, but I'd talk to the CRA/IRS first to find out exactly what you need to put down on the forms.
It is a somewhat complicated situation, you may want to talk to a good cross-border (i.e. Canadian) accountant about it if you're really confused.
One thing you will discover after you've paid out more income tax to the CRA is that moving to the US will work out cheaper, because effectively at the moment (if you're a resident of Canada) you are paying the combination of Canadian income
taxes plus US social security tax, which is higher than CPP contributions.
Really in this situation it boils down to healthcare - if you're healthy and don't need it, living in the US is going to be cheaper. Or your US employer has a really good healthcare plan then also living in the US is going to be cheaper. If their plan is rubbish and you've got some sort of problem then paying the extra tax in Canada and living in Canada to use the Canadian system may be worth it. Depends on your tax bracket to some extent.
Steve.