I like physiology a lot, and I had a great physiology professor for a class, so one day after lecture was over I literally just went up to him and asked him if he had an open position for an undergraduate researcher. I would suggest, if there is a professor whose subject matter you really like, that you visit them during their office hours, ask them about their research track, and let them know that you are interested in what they're doing.
Most professors, even if they don't have room for you right then, will be excited to find an undergraduate that is interested in doing research, especially in their field.