A W-4 is used to determine the amount to withhold for State and Federal Income Tax. You file this form with your employee. They have copies of the form that you can fill out.
The amount of FICA Tax (Social Security and Medicare) is based on your income. It cannot be changed.
The rate for an employee is 6.2 percent on earnings up to the "wage base" of $128,700 for tax year 2018.
If you want to withhold more income tax from your paychecks, so that you don't owe as much when you file your tax return, ask your employer to let you update your W4.