You need a mono amplifier, or a one channel amp. or you could get a two channel amp that allows you to bridge the two channels. If you have a low wattage sub there are also four and five channel amps that may work, by allowing you to bridge the two back channels, or by providing a separate sub channel.
The easiest route would be to just use a single channel amp designed for a subwoofer, with internal low-pass and high-pass crossovers. crossovers fade out the signal above of below the frequency that you set them to. a low-pass lets the signal through below what it is set at, and a high-pass let through the signal above what it is set to, so you can select the frequency range that your sub can reproduce well, this way you are not hearing your sub try to reproduce frequencies above or below it's capabilities (which sounds bad, and can cause damage).
With a single channel amp designed for a subwoofer you would need to run a power wire (with a fuse) from the car battery to the amp, with the gauge and fuse rating depending on the amp's wattage. you would also need a ground wire from the amp to the chassis of the car, the same gauge or larger than the power wire. you would then need an RCA cable from the amp to the head unit's subwoofer output, or if it does not have one it should have a rear channel output that will work, however you will need a Y-cable to connect it to both (left and right) rear channel outputs (they have them at radio-shack, and audio shops). then you will need what is called a remote turn on line, or just remote. It supplies a very small +12v current to the amp when the radio is turned on, there will be a blue wire labeled remote on the back of the head unit for this purpose, however if you have the power antenna you may use the line from that instead of running a new one, if you go that route just ask and i'll find out the wire color for you. Then you need speaker wire to the sub, if it is a single coil sub you would just connect it like a normal speaker, if it has more than one coil, you will need more help with that, but until we know the sub and head unit model numbers, it's hard to help any further.
It would be wise to let us know the sub model number before you buy an amp, not all mono amps will work with all subs, besides wattage their is also a minimum load rating, for instance the amp that i'm running can power a .5 ohm load, with a 1ohm load being covered by the warranty, but that is not standard, most car sub amps are warrantied for a minimum of a 2-ohm load, and may let out magic smoke if you try running a lower ohm load. there are even some that cannot power anything lower than a 4ohm load. The power ratings of the amps also change with the ohm load. I'll use mine as an example again, it is rated to put out 1500watts RMS @ 1ohm, but with a 2ohm load connected it can only supply 800w RMS, and with a 4ohm load it's even worse, only being able to supply 400w RMS.
You must also pay attention to the power rating, there are two different ratings there is the peak, or max wattage, and there is RMS, or route mean square, (think of this as average continuous output) The peak is almost meaningless, that is the absolute maximum power that the amplifier or sub can output for much less than a second before it catches fire. it is almost always ONLY a marketing ploy, because a bigger number sells better when you have two amps sitting on a shelf next to each other. The RMS is a much much more accurate rating.
you may find this site helps you understand things better:
http://www.bcae1.com/
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