The mortgage payoff has nothing to do with calculating the gain or the tax on the sale of your home. You don't take the mortgage payoff into consideration at all.
You have to determine you basis for the home. Your basis is basically how much you paid for it when you bought it, plus the cost of any improvements that you made. TurboTax will help you calculate your basis. (If you acquired your home other than by buying it, such as an inheritance or gift, TurboTax will guide you on that also.)
Your gain is the selling price minus your basis and closing costs. The gain is the amount that you pay tax on, but you may be able to exclude some or all of the gain from tax as xmasbaby0 explained above.
Follow the instructions that xmasbaby0 gave you to enter the sale in TurboTax, whether or not you got a 1099-S. But if you did not get a 1099-S and you are able to exclude the entire gain, you do not have to report the sale on your tax return. In that case, TurboTax will ask you whether you want to report it.
