How to "apply" for an internship?

I've taken a real interest in computers as of late, and am quite taken with the idea of being a part-time intern(paid or not) for a local computer repair and retail company. Naturally, the whole point of this is to learn as much as possible about computers, and I'm hoping that the company would be willing to give me some hands on experience in exchange for some free labour, which I'd be quite willing to provide in exchange. The thing is, I'm not exactly sure how to go about doing this.