I play online poker in NJ (Where it's now legal). I expect my profit this year to be between $10,000 and $20,000. Do I simply report that amount as "gambling winnings", and roughly what percentage will I owe (If that can be answered)?

The sites you play at should be sending you a 1099G at the end of the year. They are required to by law if you have winnings greater than $600. You enter that amount as gambling earnings. You can offset whatever amounts you get on 1099G with provable gambling losses.

The same websites that provide the Form 1099-G will also provide you a statement of what you initially deposited as gambling losses.

Also, if you gamble at brick-and-mortar casinos, the use of your player's card allows the casino to track your winnings and losses as well. In January, 2015, they will provide you a win-and-loss statement if you ask for it.