The stage is where slides and specimens will be placed for observation. Your stage may be stationary or move up and down to focus on a stationary objective lens. This doesn't matter as much as the double-axis controllable positioning on high-powered microscopes. For higher magnifications, slight movements can launch the specimen out of view, so holding it perfectly still is paramount. To start off, firmly position your specimen above the light source and anchor it with slide holders. Before focusing, familiarize yourself with the focus knobs.
While looking through the eyepiece, adjust the coarse focus knob until you think it is as focused as it will get. Then, just like the name says, use the fine focus knob to do any fine-tuning that your image might need.
This piece of microscope jargon refers to the vertical distance that the stage, arm, or tube can move for focusing. Combined with an objective's working distance, it determines the size (in thickness) that a viewable specimen can be. Weaker microscopes are much more generous in their working distance, enabling microscopes like the stereo microscope to view stamps, coins, and similar items. High-powered microscopes will have a more narrow depth of field.
Common working distances are 50mm for stereo microscopes and as small as 0.1mm for high-powered oil immersion objectives. As the magnification, resolution, and numerical aperture of an objective increases, your working distance will decrease.