Do I have to pay US taxes or Canadian or both?

FICA: If you are an employee working for a US company or an affiliate in Canada, you would be exempt from paying into the Canadian system if you work their for five years or less. You would pay into the US system. Otherwise, you pay into the Canadian system and not the US system. If it turns out that you pay into the Canadian system, then upon reaching retirement age, the treaty allows each country to use credits earned in the other country to determine your entitlement. You would have to read the treaty to see how this works. Depending upon how long you have paid into each country's system, you could be entitled to payments from both countries.

Income tax: As a resident of Canada, you are subject to Canadian tax on your worldwide income. Canada's system is based on residency, not citizenship. As a US citizen, you are also subject to the US tax system. Therefore, you must file a US tax return. To avoid double taxation, the US will either allow you to exclude your foreign earned income if you pass either a bona-fide resident test or physical presence test. See IRS Pub 54 for how you qualify and the amount to exclude. In lieu of excluding income, you are allowed a foreign tax credit for taxes you pay to Canada on income subject to US tax. The credit can not be larger than the amount of tax owed to the US. In other words, you wind up paying income tax at a rate equal to whichever country has the higher rate. There are separate rules for how other types of income other than wages get taxed and the rate of tax paid. There are also rules relative to payments you make into a Canadian pension plan other their social security system. Therefore, it is in your best interest to higher a tax professional for at least your first year. Have that person explain to you how the tax returns for both countries were prepared. It is possible that you would then be able to prepare your own thereafter.