Most kegerators are designed to use a commercial tap (called a sanke as HSM pointed out.) If your tap looks like this it is designed for sanke kegs:
(thanks to morebeer.com for the image)
This tap pushes CO2 into the keg through one connector (I think the one on the right side) and the beer comes out the other connection (the top in this case.)
Most homebrewers don't use sanke kegs largely because they are mostly available in 7.5 and 15 gallon sizes and most homebrewers brew in 5 gallon batches.
You will also want to get at least 3 sets of gas and beer connectors. AT morebeer they're about $6/connector so that's $35 dollars. You'll also want gas and beer line. I buy mine from mcmaster-carr mcmaster.com they're cheaper than the beer places but you can get it from any of the online shops too, I think AustinHomebrew.com is the cheapest right now.
Pepsi and Coke used to use 5 gallon stainless kegs (called corney or cornelius kegs) to deliver their syrup to fountain accounts. Now they use 5 gallon bags stuffed in a heavy duty cardboard box (called, creatively enough, bag in box.) These are the kegs most homebrewers use now. They use 2 separate quick connections for beer and gas rather than going through a single port. The tops on the corney kegs are much larger too so they're easier to clean. Unfortunately Pepsi and Coke didn't use the same type of connectors but the Pepsi kegs are by far the most popular. These use ball-lock connectors, one size for gas and a slightly different size for beer. Here is a picture of a ball-lock connector:
(again from morebeer.com)
And here is a picture of a Pepsi corney keg from kegconnection.com
Your kegerator should hold 2 or 3 corney kegs. The 4 for $80 deal at keg connections is a great price even with shipping. You'll want at least 3 so the 4 keg deal is the way to go.
You also want to get a manifold for your gas line so you can feed multiple kegs from one CO2 tank/regulator.
Finally you will want to get 2 or 3 more faucets (assuming your kegerator only has one faucet.) This way you can serve from whichever keg you choose without having to switch around tubing. If you should have room in your tower for one more nice faucet. I recommend the Perlick 525SS austinhomebrew has a
great sale at $24 each and they offer flat rate shipping so if you buy your other supplies there you'll save on shipping. You will also want a couple "
picnic faucets" which are the types you see on rented keg taps, they're cheap and you can just leave the attached to the kegs you don't have running to the tower.
So, here is what you need.
Homebrew Kegs (get 4)
Homebrew keg connectors (get 3 or 4 sets)
A manifold to split your CO2 feed
Picnic faucets (3 or 4, you can get one nice to add to the tower if you'd prefer)
Beer and gas line
You could invest in quick disconnects for one of the gas lines and the beer line to the tower so that you can easily switch between the sanke and homebrew taps. This way you would always have the ability to buy commercial beer on tap.
http://morebeer.com/view_product/17398//Beer_Gas_Tubing_Quick_Disconnect_-_In-line_W__Shut-off_Set