If you do writing on the side as freelancer any money you make will be part of your taxable income but you will be and independent contractor or self-employeed. I get a little confused about the difference, as far as State and Federal, and local tax authorities consider the difference between being self-employed and being an independent contractor. I think you would be considered and independent contractor unless maybe you decided to write poetry and sell it on the street. Here’s something on the difference. http://biztaxlaw.about.com/b/2009/04/26/self-employed-vs-independent-contractor-what-is-the-difference.htm
And Since you don’t have and employer withholding taxes, or deducting medicaid taxes or paying your social security on your freelance income you will responsible for taking that into account, and will be responsible for paying your own SS. I believe you also might have to pay some kind of self-employent tax on the freelance income. I believe if someone pays you more than $600 over the course of the year they will be required to file a 1099 form with the IRS, Stage and local treasuries. I know you guys get taxed up the wazoo in NYC. Do you live and work in the city? Will you have to pay a city wage tax. I don’t know what would happen if you live in the city but do your freelance work outside the city or in New Jersey or something like that. That could get complicated. Or if you were do do your freelance writing out of your apartment, if what expenses might be deductible, depending on whether or not you set yourself up as some kind of business entity.
But @DrBill is right. If you are hired as an “official” part time employee for your outside work you won’t really have to do much of anything. You’re employer will file w-3s, I guess they are, with the taxing authorities and you will get w2s from your employers. I had 4 part-time jobs once and I just filed four sets of W-2s with my taxes. I also once had a part-time job and worked as an independent contractor on the side and that kind of turned into a mess, of my own making, I will admit. My contractor job was working for an accountant and she kept warning me. But, when tax time came, it was a pretty big shock how much that all adds up to, when your taxes aren’t being withheld, Medicaid taxes aren’t being deducted and you have to pay your Social Security on that income entirely by yourself, no contribution from your employer. Be mindful of that. That’s all I can say.