I'm assuming you are referring to a black mirror which is called a scrying mirror. These are used for divination purposes by gazing into it like a crystal ball.
To use your mirror, cleanse it by washing it with some salt water. Then wipe it clean. Now set a candle near by so that the light shines on the mirror giving you a good surface to scry. It is good to dim the lights so not to get as distracted by things being reflected in the mirror. Now take a few deep breaths and begin to focus by staring into the mirror. Don't over focus though as it can hurt your eyes. After a while the mirror will start to look smokey and images will come. Don't force any images as it is easy to see what you want to see. Just allow the images to come as they wish.
You can ask simple questions to receive answer to or just see what the universe wants to show you. I've even asked for images to be given to me by spirit guides and ancestors with wonderful results. However, don't expect the images to always be clear. They might be fuzzy and symbolic in meaning. This is where a dream dictionary can come in handy for looking up meanings. But there are some out there that can scry so well it is like watching a black and white tv for them. It all takes some practice.
Good luck using your mirror!