As you approach the dam, you'll see a multi-leveled parking facility on your left. Park here and take the walkway to the Visitors Center.

There are numerous reputable tours that go to Hoover Dam and Lake Mead. Here is a partial list :

Black Canyon River Raft Tours: (702) 293-3776
Picture Perfect Tours: (702) 631-2608
Gray Line Tours: (702) 384-1234
Las Vegas Helicopters Inc.: (702) 736-0013
Las Vegas Tour and Travel: (702) 739-8975

Be prepared for more "dam/damn" jokes and puns than you thought possible: "Welcome to the dam tour, I'm your dam guide"