If you bet on baseball games and win money, those winnings are fully taxable and must be included as income on your tax return.
The agency where you placed the bet is required, as the payer, to issue a Form W-2G, Certain Gambling Winnings, if your winnings are at least 300 times what you bet and over $600 total (or otherwise subject to federal income tax withholding). You are still required to claim the winnings as income even if the agency isn’t required to issue a Form W-2G.
If the agency doesn’t withhold tax from your winnings, you may need to pay those taxes yourself in the form of estimated tax payments.
To report your gambling winnings, enter the amount under “Other Income” on your Form 1040. If you had any gambling losses, you may be able to deduct those losses on Schedule A, Itemized Deductions.