yep, tax rates are marginal in Canada. Refunds and payments depend on the tax braket that you are in, and how much you have taken at source (on your paycheck). When you pay taxes on your paycheck you are basically pre-paying your yearly income taxes based on an estimate. If you pay too much, you get money back when you file. If you don't pay enough, you pay more when you file.
There are free tax programs out there you can download, and plug the info you have in now, see what you get. It will give you a pretty good picture of what you might owe/get as a refund.
-- never used it, but I'm pretty sure it's free.