If I want to withhold taxes from her payment, how much and what should I deduct? If I will not deduct taxes from her payment, do I need an EIN if the total payment per year is more than $2000? thanks
If you issue a W-2 then you will need a EIN.
You should read IRS Pub 926 (Household Employer's Tax Guide) for a lot of information about withholding and W-2's
Also Pub 15 for withholding tables and other information for employers.