Should I input my gross or net commission earnings for taxes? I am a real estate agent and I made a gross commission, about $15k of that gross was paid to my brokerage.

You would report your gross as income, and then any expenses you had (such as your brokerage fees) as an expense on the return. As you navigate through the Turbo Tax software will guide you through where to enter in your expenses in to set categories. It will also help you create expense categories that you need to. 

Your gross is the amount reported in Box 7 of your 1099-MISC.  If the money you paid to the brokerage was taken out of those funds, you can deduct it as an expense.  But if the broker's "split" was not included on your 1099-MISC, you can't deduct it.