I live in utah and want to know what portion of my car registration fees I can enter as a deductible. There's county fee, sales tax and state fee.

All.

On your federal return in Deductions & Credits. 

In the section Cars and Other Things You Own

You will combine the county and state fees to include in Car Registration fees

In Estimates and Other Taxes Paid

Enter the sales tax you paid

This will import to your Utah tax return for the deductions.