A ten gallon tank should be sufficient in size as a sump for a 55 gallon. If you have the money/patience, I highly recommend that you buy a tank that already has an overflow in it.
An overflow looks like a vertical, rectangular box on the back of the aquarium. It has holes drilled near the top, just below where the water line would be. The water flows down the overflow underneathe the tank into the sump.
The sump generally has a filtering system, a heater, a protein skimmer, and a pump to return the overflow water back into the main tank. The amount of water overflowing down into the sump, and the gph of the pump should be set to the exact same rate, oviously.
You can have a system as simple as that, or a much more complex one depending on how much money you are willing to invest. The most impressive/expensive systems I have seen utilize a series of canisters that each have their own purpose and are connected by pvp pipe. One cnister may be mechanical filtration, one may be chemical, one may be biological, etc.
The only advice I have is if you are going the sump route, don't skimp on the prices. There's really no reason to go through all the trouble of setting up a sump if the equipment is sub-par and inefficient.
I would have loved to have had a sump, but I got a great deal on a tank/stand without preassembled overflows
I'm sure there are great threads on these forums if you do a search. Unfortunatly most posts that have links to offsite information generally get moderated here.
Hope this helped some