If you have a complete 2600 or 5200 then you are in luck as you've got on your hands a real classic. Here's what you need>
Firstly: the actual Atari unit
Secondly: Joystick(s)
Thirdly: Carts (simply the software cartridge)
Fourth: an RF cable (directly into the back of the TV) You may need to retune TV until you get a picture
...Lastly (but by no means least) a Power Supply...
Anything missing from the list? If you have everything on the list then there's nothing else required.
Does the unit power up? You tuned the TV in properly? Is the cart needing cleaned? (These units are pretty rock solid and reliable, however as they area a few years old now you might find it has picked up dirt and dust on the actual contact points so give it a clean - be careful not to use any cleaning fluids that might get into the unit and causes damage!!)
Finding someone with a bit of technical knowledge (and I mean not much) and your pride and joy should be stripped and cleaned inside 20mins!
Hope you get it going as it is well worth it, although I suspect most kids these days will wonder what all the fuss was about - they have PS II's and Gamecube's remember!