One thing I would sincerely recommend if you are holding out for the PetCo $1 per gallon sale is to get a 40B instead of a 55. It has more surface area (roughly 648 sq inches vs. 576) and with the extra depth front to back, it ends up being much easier to decorate. If you're going to do corals or anemones, it's also easier to light.
Live rock is a mixed blessing. You get lots of cool stuff, but you are also likely to get unwanted hitchhikers. Uncured liverock as Mebbid said will help you cycle and ultimately establish a stronger bacterial bed, but often stinks pretty bad as it cures. You never want to add it to an established aquarium. Dry rock is cheaper and safer. Live sand is also an unnecessary added expense.
If you want a bigger tank, craigslist or your local reef / aquarium club is the way to go. there is almost always someone moving or getting out of the hobby and you can get a much better deal than retail.
As far as first fish, ocellaris clowns are pretty good as Mebbid said. I'd also consider chromis (NOT damsels), firefish, cardinals, a royal gramma, or one of the several species of blennies. I also am very partial to flasher and fairy wrasses.
Fish I would avoid as a beginner include butterlies, mandarins, scooter dragonets, and cleaner wrasses. These often have trouble adapting to prepared foods. Unless you get a larger tank, avoid tangs as well. Avoid damsels becasue they are often aggressive. Once established, you might consider a dwarf angelfish, but be warned they sometimes decide to eat corals.
Two more pieces of unsolicited advice: either buy purified water from a tested source or make your own with an RODI
system. Tapwater is algae fertilizer.
Secondly (and finally), invest in a quarantine tank. It doesn't have to be anything fancy. in fact, I use a 10 gallon with a HOB
Aqua Clear filter and a powerhead for mine. Any new fish go in for at least two weeks to be sure they aren't carrying anything which will infect my established fish. Effective treatments for most fish diseases are fatal to invertebrates. treating a new fish in isolation is cheaper and easier than catching an infected fish from an established tank.