science and how to put sound to lights....

I hope someone can help me out on this one!

I got the idea in my head that I wanted to put the beats of music to lights. When I turn on my radio, I want the lights to flash according to the music. After talking to my father, he said this was done with what used to be called a “Synthesizer” that used to be available at Radio Shack… Well, I called up there and they did not carry anything like that.

What can I do to make this possible? Will it be a costly project? I’m a novice in this field. Thanks for the help!