It will be one of the best and most memorable things you'll ever do.
We just signed up with one of the advertised tour companies, which involved being picked up at the hotel, driven to this huge bus terminal where everybody taking any kind of tour was directed to the appropriate booth to pay for your tour and be directed to your bus for whatever tour you were taking. We rode the first bus for a while, which made a stop somewhere I don't recall (Boulder??)where there was a lookout point and of course a gift shop. Then onto another bus which took us to the drop off point at Hoover Dam. Folks using their own transportation don't have to go through all this and probably get there in a shorter amound of time, but it wasn't unpleasant. For newbys like us it was good not having to figure things out on our own as the bus drivers were great to point things out and tell us what to do.
Before 9/11 they had what they called the Hard Hat Tour. After the 'orientation' and viewing of a short movie about the history of the building of Hoover Dam, they gave us cute blue plastic hard hats which I think were mainly of souvenir quality and probably for effect. Supposedly on the Hard Hat Tour we were taken farther down into areas that the 'regular' (cheaper?) tours did not include, but again, not sure of the truth of that.
Do view the movie Vegas Vacation (Chevy Chase) before you go. There is a major Hoover Dam scene in that movie, very funny, and you will get more out of the movie the next time you watch it after you take the tour, plus get a little more out of the tour if you have seen the movie beforehand.
Enjoy the tour!