Best bet would be to find a proper tax advisor. It's a little involved.
Your capital gain is the difference between the sales price and your basis. Basis is what you paid for the property adjusted by any capital improvements made. You can also deduct some of the direct costs of the sale (real estate commision, repairs to put the place on the market).
That's all relatively straight forward, but here's where it gets tricky. The basis is decreased by the amount of allowable depreciation that you were entitled to take on the rental use, EVEN IF YOU DIDN'T TAKE IT. That part of the gain is also taxed at a (possibly) different rate than the rest of your gain. Best to let someone who does this for a living take care of it.