Where to claim the clergy mileage depends on where the income originated from.
If you only received a W-2, you are considered an employee for the amounts listed on the form. For any related expenses to this income, any unreimbursed business expenses will need to be deducted as an itemized deduction on Schedule A.
If you receive any additional offerings or fees from performing marriages, baptisms, funerals or related, those may not be on your W-2 and are considered income from self-employment. They are reported on Schedule C.
This link provides more information on clergy income - https://www.irs.gov/taxtopics/tc417
See links below that explains how to enter job related miscellaneous expenses (which includes mileage)
Where to enter Schedule C