Filter for a 55g: A Fluval 304 works wonderfully. Bought at
www.bigalsonline.com you can save quite a few bucks. They're twice as expensive at my local fish stores than there. They provide mechanical, biological and the option of chemical filtration and are more easily adjusted. You can control the flow and the positioning of the intake and outtake tubes.
Heater: 150w - 250w will be just fine. Visatherme and Ebojaber make great heaters, but they can be a little pricey. I think I got my submersible, adjustable Visatherme for my 55g for 25$.
Cycling in a BRIEF nutshell: Fish poop. This turns into ammonia (the stuff you can clean with). Ammonia is very harmful to fish. Certain bacteria form in your filter when ammonia is present. They "eat" it and turn it into another chemical called Nitrite. Nitrite is also *very* harmful to fish. A second group of bacteria forms in your filter, along with the other bacteria and they convert nitrite to NitrAtes. These are much less harmful and are easily removed with a water change once the levels get past 20-40 parts per million. When you cycle a tank with fish, the fish are subjected to these levels of ammonia and nitrite and can kill them. To bypass harming the fish (and having to buy "hardy" cycling fish, only to return them later) you can bypass the fish and just make fish poop in the tank. How do you do this? You can add pure ammonia - afterall, that's what fish poop is. Once you have cycled the tank and the ammonia and nitrite are down to 0, you can add all of your fish

But as OrionGirl said, check out all the threads in the Newbie Section
Your fish choices sound nice - not too sure about the shark though. Even the "peaceful" ones tend to cause a few problems.
Brett- No, you don't need two filters. There are instances where this can be helpful, though. IN the case of a filter problem, you have an extra cycled one on hand. If you need to start another tank or a hospital tank, you've got one on hand. However, to just run the fish tank, no, one adequate filter is enough.