what form do I use to deduct cost of an elevator?

An elevator for what?   Where did you put an elevator? Provide some details.

You can deduct it as medical expenses if its main purpose is medical care for you, your spouse, or your dependents. However, If you itemize, you can deduct only the part of your medical and dental expenses that exceeds 7.5% of the amount of your adjusted gross income.

If it is a capital improvement  that adds value to your home, you won't be able to deduct it. It can be added to the cost basis of your home and subtracted from the sales price to determine the amount of your profit when you sell it.

what form do I use to deduct cost of an elevator in my home to go from floor to floor ?

Medical expenses are reported on Schedule A and entered in the Deductions & Credits section.

1. With your return open, search for Schedule A and then select the Jump to link in the search results.
2. Enter your medical expenses, starting with prescriptions, on the following screens.