Would I need to fill out:

New Jersey NJ-W4 - resident

As well as:

New York IT-2104 - resident
And
New York IT-2104.1 - nonresident?

Furthermore, I'm not sure how many allowances I need to write down on each form. I am single (just sharing an apartment with my girlfriend), no kids or anything like that.

I generally use TurboTax at the end of the year to file the annual returns, if that matters any.

No need for NJ-W4, unless you want your employer to withhold taxes for the state of NJ. Assuming that the NYC job is your main source of income, it's unlikely that you will actually owe NJ any income tax, so there's really no need to have any taxes withheld for NJ. If next April you find that you do owe NJ income tax, because of NJ-source income such as investments, you can start filing quarterly estimated tax forms with the state.

Yes to IT-2104 - it's required for anyone working in NY State
Yes to IT-2104.1 - required for non-residents

Follow the directions for determining the number of allowances. Your filing status is Single, and for IT-2104 you start with 0 allowances. For the federal W-4 you start with two allowances - one for yourself and one for being single with one job.

On all the forms, claim SINGLE with ONE allowance. That will cover your tax liabilities.

I don't know why my employer is asking for all three forms, then?

Maybe it will be easier if I am more specific:

http://www.state.nj.us/treasury/taxation/pdf/current/njw4.pdf
Question 1: Do I need this form?
Question 2: What do I put on lines 3-6? Is it (blank, 1, blank, blank)?

http://www.tax.ny.gov/pdf/current_forms/wt/it2104_fill_in.pdf
Question 1: Do I need this form?
Question 2: What do I put on lines 1-2? Is it (blank, 1)?

http://sfs.columbia.edu/files/sfs/content/it2104_1_fill_in.pdf
Question 1: Do I need this form?
Question 2: Do I say "I certify I am not a resident of NYS..." with 100% services in NYS, not a resident of NYC, not a resident of Yonkers, 0% service in Yonkers?


All relevant background:

As of this very second, I live in NYC, and work in NYC, and have been all year. Tomorrow, I am moving to NJ, but will still be working at the same firm here in NYC. I am "single," just living with my girlfriend, who is also moving with me.

I am under the impression that I need to send new forms to my employer so that taxes are handled properly going forward.

I am also under the impression that TurboTax will automatically handle any complexities at year-end when I file (it's gonna have to because I have no effing clue how taxes work and how NYC/NYS/NJ play off each other).

I sais no to the NJ W4 - reason is that if you complete it your employer will have to withhold both NJ and NY taxes from your pay. But as I said befire - you won't actually owe NJ any income tax. So don't submit this form.

For IT-2104 based on what you said your allowances are 0. Put a 0 on lines 1 and 2.

For IT-2104.1 - yes, you certify you are not a resident and 100% of your services are done in NY.

And FYI -though you didn't ask about it, come next April when doing state income tax returns you will be filing a part-year resident return with NJ and a non-resident/part-year resident return (Form IT-203) with NY.

So I don't put an allowance of 1 anywhere, pretty much? 0 all the way through?

(does this not imply that I can be declared as a dependent, or is that something saved for the year-end federal return stuff?)

No, putting 0 allowances does not imply that you are someone else's dependent. Have you followed the directions on Form IT-2104? What did you enter for lines 6 - 17?

All 0's -- only entered 7800 on line 23 otherwise, didn't seem to impact anything else.

(I wasn't sure if I should also put 0 for 14 -- I only one job but "head of household"?)

You are not "head of houshold," you are "single." So indeed it's all 0's.

So by submitting the IT-2104 forms (both of them), the net effect should be the removal of the NYC tax from my paychecks, since that only applies when living in the city? (to my understanding)? Nothing Jersey-related should show up?

Correct.

Great. Thanks for all your help!