How to loosen up enough to dance?

Recently I was at a wedding and I wanted to dance, but I was too shy. I am introverted by nature. When I tried dancing, I just felt REALLY stupid. I don't like to just bend my knees the whole time; I want to bust out different moves that hopefully don't look too stupid. How can I cheaply learn how to dance in situations like this and how can I be comfortable dancing? Besides watching music videos I don't know what I can do to know different moves. By the way, I'm under 21, so don't recommend drinking alcohol to me. I'm also not into drugs.