When buying a bass guitar, you will also most likely be purchasing an amplifier, or amp for short. The amp is essentially the thing you plug your bass guitar into to make the bass loud enough to hear. Any store that sells bass guitars will sell amps, too.
Amps come in every possible combination of size, power, loudness, sound, and look; there are are many more variables than can be addressed here. However, here are a couple of tips:
-Amps are usually categorized by wattage (electrical power), speaker size and the number of speakers (although most entry level amps only have one speaker). The wattage will usually be somewhere between 20w and 500w, and the speakers between 8 and 15 inches.
-know what you are are going to be doing with the amp. If you are just playing in your bedroom, pretty much anything will work, although you may want to try to find an amp with built in “practice” features: A headphone jack and a way to plug in an external audio source (music player, computer, etc). If, on the other hand, you think you may be playing in a band or performing before too long, know that an amp with less than 100 watts of power and a speaker smaller than 10 inches is really going to struggle on the bandstand, and probably in rehearsal as well. Remember, though, you are going to have to carry the thing to said performances and rehearsals, so don’t get way more than you need, either. Generally 150w-200w and one 15 inch speaker (or two ten inch speakers) is enough for your average rock show.
If you want to go into greater depth, here are some more links:
http://www.tunemybass.com/bass_amps/bass_amp_basics.html
http://en.wikipedia.org/wiki/Bass_instrument_amplification