You will fill out a Form W-4 when you begin a new job.  Additionally, each time you experience a life change such as a new address, marriage or a babys birth, you need to update your W-4 promptly. 

Do not wait until the end of the year as you may experience an unpleasant surprise in how much you owe in taxes.  If you have problems with the form your HR staff can help you. You can also contact your tax accountant for assistance.