I know this post sounds moronic -- like I said, I am new to this! Any help you could provide would be most appreciated! Thanks!
Its not a dumb questions because you will encounter quite a few nannies who want to be aid under the table or if they must do taxes want you to pay everything and guarantee them a net. Don't do this! There are plenty of more professional nannies for that rate who understand taxes.
You want to negotiate and contractually guarantee the gross ($18) amount, not cycle up to guarantee a net amount. You can create a sample pay stub to show your nanny what this year's withholding would be from her pay stub.
As an employer of one household employee you are only required to withhold SSN/Med not state and federal income tax. If you are good with taxes and organized, you can offer to do this for the nanny but understand that she must provide with her deductions etc. You have no idea what her additional may be and you don't have enough info to know which deductions she should take. If you are not comfortable doing this, you are not required to do it. Most nannies seem to want the most in the check and prefer not to withhold anyway. Not sure why but it is what it is.