A PA is a big amplification system. You've got a sound board, which you plug your mics into, then your power amp, which amplefies the signal for the speakers, and then the actual speakers, which make the noise.
Anywhoo, I use a 15 and its loud enough, so if you want to avoid spending extra money, tell everyone else (especially your drummer, drums are very loud) to play quieter, if that still doesn't work, a 30 watt amp should work fine, but if you wanna play live without a PA, go with a 50 or higher.