Where do i enter my mileage for my vehicle on turbo tax?

If you are self-employed as an independent contractor you can enter mileage on your Schedule C.  You are posting your question from online Deluxe---which does not support preparing a Schedule C for business expenses.  If you have business expenses you need to upgrade to Premium.

In the Business Expenses section, there is an Expense Category for Vehicle.   Follow the below instructions to enter your business expenses in TurboTax Online.

1. Type Schedule C in the Search box.
2. Select the Jump to link.
3. Answer Yes to Did you have any self-employment income or expenses? and answer the questions until you get the Select any expense categories you have screen.
4. Select your expenses, then Continue.
   • If you've already worked in this section, select Edit or Review next to your business.
5. Then select Start or Edit next to an expense type that you had.  One of the options is "Vehicle". 
6. Enter your expense description and amount, and answer any other questions we ask.
7. If you had more than one expense for a type, select Add another row to include them all.
8. If you have additional expenses of other types, select Add expenses for this work and repeat steps 4 through 6 to add more.