How do I determine how much to write off for cell phone use if I am self employed and have one phone for personal and business use?

You will need to make a reasonable allocation of your personal use and your business use of the phone.  For example, if your overall use is half business and half personal, you can deduct half the monthly cost of the cell phone on your Schedule C.  If your business use is only 30% of the total use, you can deduct 30% of the monthly fees, etc.