your chevi impala was built in several audio options. If you have a lower model, you most likely have a radio and 4 speakers. The higher models had a factory amplifier. The correct advice on your vehicle is to pay a professional installer to wire up your radio for about 100-150.
If you want to do-it-yourself, here is the plan.
You want to have 3 parts, Mounting kit so that the radio will fit in the dash properly, Wire Harness so that you have the correct wiring and Antenna adapter because a GM radio does Not have a universal antenna plug size. All 3 parts should cost about $30 total.
The wire harness will have the univesal wire colors on them and plug right in to the factory wire harness. Then its a mater of putting black to black, yellow to yellow, red to red and so on. (your gm probably does not have a Red wire attached. You will need to go to the fuse box or under you steering colom dash area and Tap an Accy wire. This wire has power when you turn on the key and off when you remove the key, it makes your radio turn on and off.)
once you have the radio wired up, make sure you ground the light green parking brake wire, (attach to black wire is easy way to do this.) This will let you watch dvd while driving.
Also the blue wire and/or blue with a white strip will turn on any factroy amplifier you may have. Even if you dont have the amplifier, hook up this wire anyways cuz it wont hurt you car in any event.
Plug in antenna adapter and everything should work. Dont try to get cheap now and bootleg the last 30 $ in parts, you need to have these parts!