First: if you are an author, you are effectively self-employed (assuming you have no day job, which is, to be honest, a bit of a stretch). This means you may have to report your earnings to the IRS on a quarterly basis and will certainly have to pay self-employment (SE) tax (15.30%) via Schedule SE of the IRS Form 1040. (You can learn more about the tax forms you'll need here.)
Second: you're eligible for a lot of deductions as an author, particularly those that pertain directly to your writing career. Office supplies, use of your car for travel (e.g. author tours), books, magazine subscriptions, writing workshops/conferences, and dues to professional organizations (such as MWA or RWA) are all deductions you can make. You may want to look into getting an accountant to help make sure you get the greatest number of deductions possible.
Third: some authors are under the impression that royalty payments fall under the capital gains tax (rather than income tax) and are subject to the lower capital gains rate (15%). This isn't the case. (Even if it were, the capital gains tax is set to revert to pre-2003 levels—around 28%—next year, so you wouldn't be saving all that much regardless.)
Finally: speaking of royalties/payment, you need to budget effectively. Publishers take several months to calculate royalty payments, meaning you'll probably only get a handful of "big" paychecks per year. If you've got a day job, it might make sense to keep your writing income in a separate savings/checking account and rely primarily on your day job's salary to budget, pay rent, and so on. If not, you'll have to be careful to budget your money so as to live comfortably year-round, pay your taxes, and (hopefully) set some aside in savings. Again, hiring an accountant to help you go through your finances and construct a budget might make sense.
Questions? To the comments!