He won't, at least not if he really does expect Mexico to pay for it. Remember how Trump lied about not discussing the all with Mexico's Nieto, only for Nieto himself to call Trump out and say he'd rejected the idea? Yeah. Trump will have to get funding for the wall from the US, and even a sycophantic Republican Congress will probably balk at the many billions it would take to build and maintain that wall.
More realistically, you'll see more (and more advanced) border patrols. That could please some of Trump's supporters, but I suspect that a lot of his anti-immigration fans are going to be disappointed that his pie-in-the-sky dreams can't become reality.