Greetings. For a single keg, as you already know you need the keg, CO2 tank and regulator. Your keg should come with its lid and posts intact, complete with new O-rings. To get the gas into the keg, you need a hose from the regulator to a "quick disconnect" fitting on the "in" port on the keg. To get the beer out, you need a "quick disconnect" fitting for the "out" port, connected either directly to a beer faucet, or to 5-6 feet of a narrow-I.D. beer line (3/16 inch is best, but 1/4" can work for longer lengths) to a faucet of one form or another. Depending on your serving desires, you can use a simple hand-held "cobra" or "picnic" faucet, or go with a more formal beer faucet mounted on a fridge door or kegerator.
There are two types of corny kegs, "ball-lock" or "pin-lock", which describes the type of fittings they use. They require the correct type of disconnect to go with the type of keg fitting (either ball-lock or pin-lock), and either type also requires a separate gas and beer connector because they are not interchangeable.
Quick disconnects are also of two broad types, either threaded or "barbed". The barbed fitting accepts the hose shoved onto it directly, whereas a "threaded" fitting accepts either a threaded "barb" or a fitting to attach a beer faucet directly. Some people (me included) prefer threaded disconnects, because you can take the barb off to more thoroughly clean/sanitize them, particularly the liquid line. Being able to remove the barbs on a threaded disconnect also allows the flexibility to use the same disconnect with different sized barbs for different diameter beer or gas lines.