The first question you should ask before buying a microscope is, "What do I want to observe?" This determines the type of microscopes you need, as well as how to prepare the subject for observation.
Very small subjects - cells and microorganisms - require a lot of magnification and you will need to prepare a glass slide to observe them. The slide is then used with a compound light microscope at magnifications anywhere from 40x all the way up to a maximum of 1500x, depending on the subject. The compound light microscope is the most familiar type of microscope and the one people think of when you mention the word "microscope". This is the microscope we first used in science class. A glass slide with the specimen is placed on the microscope stage and is first viewed at low magnification, then at higher magnifications by rotating a nosepiece which has 3 or 4 objective lenses attached.