I need help buying cleats for my bicycle.?

I have been road cycling for about 6 months now and I average around 100 miles a week with a 30 mile ride once a week. I have always ridden with tennis shows but I am looking into getting some cleats for the first time. The problem is that I know nothing about buying cleats. If someone could give me some information about cleats and or refer me to a good website to buy them then it would be much appreciated. I am not looking to spend a ton of money but I am willing to put in somewhere around $100.