Wilco, there's lots of info stored here. Search for PA Systems, microphones, etc., in the threads. Opinions differ, but you'll get lots of good ideas.

To get you started, you will need a small PA (Public address) mixer with 6-8 channels. A channel is an input for a microphone or instument. You will need two house speakers, that is, the speakers that project the sound to your audience. You need speaker stands for those to lift them up to reach the back of the room. An important option for your performer(s) are monitors. These (usuallly there are two) direct sound to the performer so that they may hear themselves. You of course need the cords that run from the speakers to the mixer. You will also need microphones and mike stands. No, one microphone will not do. You need one for each vocal and, most likely, one for each instrument.

What I have given you is the basics for what you have described. If this doesn't scare you off you will find lots of recommendations for PAs and mikes, etc., in the threads, or ask here. One thing, don't call 'it' an amp. An amp is an amplifier for one instrument, generally, at least here in the US. cheers, Fortunato