March Madness gambling winnings are treated as taxable income, and you are required to report those winnings on your tax return every year. If the gaming organization doesn’t withhold tax from your winnings, and those winnings were substantial, you should pay those taxes yourself in the form of estimated tax payments in order to avoid an underpayment penalty.
You may not get a Form W-2G, Certain Gambling Winnings, from the agency you placed the bet through; as the payer, the gambling agency isn’t required to report your winnings unless you win at least 300 times what you bet and that amount is over $600 (or otherwise subject to federal income tax withholding). You’re still required to report your winnings to the IRS, however, even if you’re not sent a Form W-2G.
The “Other Income” line on your Form 1040 is where you’ll report your gambling winnings. Any gambling losses might qualify as a deduction; if so, report the amount on Schedule A as an itemized deduction.