Any mirror can be used as a scrying mirror. You dont need to buy an obsidian or black mirror.
To make a regular compact mirror a scrying mirror, it should be cleansed & consecrated (this can be done by salt water & incense) and blessed or charged with the task of scrying. One way to do this is to gather some mugwort, charge the herb with the intent to bring visions, and rub the mugwort on the mirror in a clockwise circular motion untill you feel it has been adequately charged.
Scrying is a method of divination that allows the practitioner to peek into the spirit realm. Sometimes you contact ancestors, sometimes you even get the gods themselves.
The messages that you recieve through scrying are usually visual cues, or evn through words or phrases that you “hear” while in the trance state necessary for scrying to be effective.
For effect scrying it is important to be in a dimly (candle) lit room with minimal to no distraction. sitting comfortably gaze into the mirror, your image may fade & shift to appear like someone or somewhere else. Keep a journal of your scrying activity, so that you can readily chart your progress. Scrying at the waning moon, and at the cross quarter days (samhain & beltane) tend to help the beginner find the right “channel”
Best of luck to you.