The band needs a PA system. This includes a soundboard, power amp and PA speakers. The size and power required depends on how loud the rest of the band is. You can get a fairly good system for under 1000 bucks, but be careful. Buy the best system you can afford. This is going to be the workhorse of your band. This is a minimum of what you will need. Stage monitors, drum mics, and other equipment like equalizers, effects, etc are not needed now, but if you start playing often, you are going to want to enhance your sound system.
Go to guitarcenter.com and take a look at some systems. Yamaha, Peavey are good starts. Behringer is sort of mid to low end, but it's reliable, sounds good and isnt expensive. You are going to need a mixer, a power amp and two speakers. You can always add on. You can also get a "powered" mixer, which is the amp and mixer together. The mixer will allow you to adjust bass, treble, etc, add some effects like echo to your voice, and add other microphones to the system. You can also put instruments throught the soundboard. Some bands will put a bass guitar through it and have the bass on stage in it's own amp. But you need really good speakers to do that.
If your band is going to be serious, you need to look at this stuff. As I said, get the best you can afford. If you cant get something now, you are better off using your guitar amp for a couple of gigs until you get some money together. If you buy crap, you're stuck with a crappy sound.
My band started out that way a long time ago. I know what I'm talking about.