Based on what the IRS has published; list all of your gambling winnings as "Gambling Income" on line 21 of the 1040. Then you list your losses as "Gambling Losses" in your Schedule A under Other Miscellaneous Deductions.
https://www.irs.gov/newsroom/five-im...nd-losses-2012
Most of this stuff you don't have to worry about unless you get audited. Just reporting the income in of itself isn't likely to lead to an audit. So for example if you report $50k in gambling winnings, it's not a big deal. It's when you have a ton of deductions on your return that reduces your taxable income to nil is when eyebrows start getting raised and the odds of being audited get increased. Declaring $1 million in overall income with $950k in deductions is when people get audited.
IMO, if you do happen to get audited, I doubt they're going to dig further beyond you saying it was winnings from sports betting (there's plenty of people who win in Vegas without records and W2-Gs) as long as you have documented your wins and losses (and I don't even think they can go after you simply for gambling; the laws are more to go after the illegal bookies).
As for the Bitcoin enigma Bsims is mentioning, if you're strictly using Bitcoin as a medium of exchange and do the exchange same day, I wouldn't even worry about reporting that and just simply tell the IRS that was your medium of exchange for your gambling funds and didn't hold any if they audit you. If the numbers correlate, I doubt they're gonna dig that deep. It's if you decide to hold onto the Bitcoin is when you need to keep track of your basis in the Bitcoin then declare your losses/profit when you have a taxable event.