When it comes to paying someone legally we recommend two providers. Breedlove-Online as a payroll service that will handle the weekly processing for you from soup to nuts AND Legally Nanny, to not only set up your payroll but help you establish a healthy legal nanny contract. Both are excellent resources to check out!
Breedlove-Online Resources included a payroll calculator: http://www.breedlove.com/Resources/Nanny-Tax-Calculator.aspx
Legally Nanny: http://www.legallynanny.com