Okay, I'm copying this from my answer to another newbie as it would seem to fit here....
The stock head unit connector on your car has: 12v line from the battery, 12v acc line, ground, lines out to the speakers (usually 4 pair), and probably other wires for various other functions.
You cannot draw your power from this connector because the wire is not thick enough for the amount of current you will have to draw, you will end up burning out the wire and possibly starting your car on fire. This is why you must run large cable 4-8 gauge from your battery. You only need to run the positive wire (+)(12v)(red) because the negative (-)(ground)(black) is connected directly to the chassis of the vehicle, so the entire metal frame of the car acts as one big ground wire. This is why you can connect the ground to (almost) any bare metal on your car.
When installing your carputer, you will need to identify: the acc line, and the 4 PAIRS of wires heading out to your speakers. You will have to either A) Tap these lines, B) Find a corresponding plug or C) Cut the connector off altogether. I suggest C. But you must make sure that you TAPE OFF ANY UNUSED WIRES (with black electrical tape) and LEAVE NO WIRE EXPOSED.
Now, in order to identify which line is which, you will need either a volt-meter, or multi-meter, or a 12v test light. Since you seem to be technologically uninclined, i suggest the test light. They only cost a dollar, and their operation is simple. You connect one end to a ground and the other end is a probe with a light in it. When you probe a voltage source, the light lights up. You could also connect the wire to a positive line and probe for a ground, but you will not be doing that.
Now, with the test light, connect the wire end (usually has an alligator clip) to some bare metal someplace on the body of the car. Turn the ignition on (engine does not have to be running), and begin probing the wires if you find one that lights up the light, then shut the ignition off, if the light goes dark, you have found the acc line. If the light stays on, it is the 12v (positive) line, but you won't be using it anyway. If it is not the acc line, turn the ignition back on and move onto a new wire, and so on, until you identify the acc (accessory) wire.
NOTE:Red wires are almost always 12v positive wires while black wires are almost always negative ground wires.
Make sure you identify the 12v positive, the negative ground, and the acc lines and exclude these wires from the next test. Also tape of the 12v positive and ground wires because you will not need them, as you will be running new ones.
You will now be finding the speaker wires. Use and AA battery and find any two of the remaining wires and touch one to each end. If you guessed right, you will hear one of your speakers make a scratching sound. Make note that this is a pair and which speaker it goes to, and continue on with the wires that are left until you have identified all of the wires going to your speakers.
Note: Stock speaker wire pairs are usually different shades of the same color, so try those first if applicable.
Tape off any wires you have not identified after this process because you will not need them.
This is it for identifying which wire is what. But you will need a lot more info in order to complete your install.