You are going to need a Power Amplifier, at least one PA speaker, and of course a microphone designed for this use. The cheapest I could put together a system that should work for your needs is approximately $470. The cheapest Power Amplifier that we sell that would work for this is the QSC - GX3 which sells for $299.99 and you can see it here:
http://www.bestbuy.com/site/olspage.... The cheapest PA speaker we sell that should work is the Peavey - PR model 00583890 which costs $149.99, and you can see this speaker here:
http://www.bestbuy.com/site/olspage.... The microphone I found that should work for you is the Samson - Dynamic Vocal Microphone model SCR10S which sells for $19.99 and you can see this microphone here:
http://www.bestbuy.com/site/olspage....
If your local Best Buy® store has a musical instrument department I would suggest going to your local store and having one of our live sound experts help you choose the right system for you. I hope this helps!