U need 4 things, really.
1. Amplifier (get a Sony Xplod Amp or Pioneer One. if ur woofer is already installed in ur car, then U should already have an AMP, take that amp with u)
2. The woofer, that u have.
3. An 12V DC Adaptor(make sure it can do it can satisfy AMPS Amperage requirement) .. U may need a 'heavy duty' one as AMPS guzzle a lot of power.
4. An enclosure made of wood, Pref. an MDF Based Enclosure, Note if ur Woofer is 'free air type' then may use a large enclosure wit necessary ventilation (so that woofer dont heat up) . If its a boxed type woofer, then u may need to get the enclosure of exact dimesions (also note if the woofer is meant to be ported or not). Not Enclosure is Not all that critical. But if u do, make sure u do it right, as an improper Enclosure may (though remote) chance that it may screw up ur Woofer, But most of the time Imporper enclosure's will only give u less than optimal output.
Note , the most important thing is the 12V adaptor/eliminator u get, as AMPS are very expensive and sensitve equipments.