I did it three years ago. First let me definitely suggest you get a few ocean dives in before doing the shark dive. If you haven't done many dives and this being your first ocean dive, you might have your hands full fiddling with your newly found positive buoyancy; having to also deal with sharks as big as 8' coming within arms reach of you, while dealing with all the other, might prove to be a little overachieveing. Second, do pay attention to the DM's dive briefing. They know what they're saying. (for some odd reason everyone tends to do that more on this dive, although it should on all)
If it's still being done the same way (and I wouldn't know of a reason why it wouldn't be), it's a two tank dive. The first dive you swim with them in their natural habitat. This can be a bit nerve wrecking, even for some of America's finest (I had 3 Army rangers in my boat and when we got to the dive site they were having second thoughts about doing the dive as there were in excess of 40 dorsal fins breaking the surface), but it's common, so not to worry. The second dive is the feeding dive; the DM jumps in first with the food and you guessed it, the sharks follow him. You just go to the sand, kneel in a semi-circle with your hands tucked under your arm pits and watch the show.
Hope you enjoy it!!