But, as others have said, the law is already on the books to build the wall.
The Secure Fences Act of 2006 authorized the Department of Homeland Security to build nearly 700 miles of double-layered fencing on the Mexican border.
That's enough for President Trump to enact an executive order to act on the existing law, a law that has been ignored by the jerkwad outgoing president. Going beyond that law can happen later, with a request to Congress to create a stronger wall.
He’ll need an appropriation for the costs.