Click on the Federal Taxes tab (or Personal in TurboTax Self Employed), then Deductions & Credits, then scroll down to Charitable Donations and click on Donations to Charity in 2016.
At the beginning of this section, TurboTax may ask you some questions regarding itemized deduction amounts, just to see if it's worth your while to itemize. Then, once you enter the donation screens, you'll come to a page that says Let's enter your donations one at a time.
Here you'll enter the organization for whom you volunteered, in the box marked Who did you donate to in 2016. Then you'll scroll down to the various choices for your first donation entry: "Items" "Money" "Stock" "Mileage & Travel Expenses." You'll select the last of those choices (Mileage & Travel) and follow the prompts to enter your mileage.
The charitable mileage rate is $.14 per mile.
