First, what is your budget?
Subwoofer selection is probably more complicated than you think. Subwoofer choice is at first dependent on the volume area of your room--including any open areas--and your listening volume. Too little sub and your sub will struggle (or be unable) to keep up with your speakers, and you might overdrive it. Now if you are using it nearfield for a computer setup (approximately 3 ft away), then the subwoofer performance based on the room size is not a concern).
You need to consider the low end frequency extension capability of the sub. For music, a good general guide is that you want extension down to 30hz. However, movies have deeper bass extension where a lot of special effects content is. Solid low 20hz (or lower) extension is a big plus for a good subwoofer for HT usage.
A sub is a big driver, in a big, heavy enclosure (box), and it has an amp. It is expensive to ship. There are significant price/performance benefits on spending more on a sub than most people new to home audio realize. When choosing a subwoofer, there are also Internet direct vendors that offer much better price/performance values than traditional speaker company subs that you can buy at a store.
Finally, what system are you using it with? (make/model components and speakers? audio sources? It's generally pretty easy to integrate a subwoofer with an audio/video receiver. But for other setups, you may need a sub with specific connection capabilities.