Gently clean the lens in the eyepiece and on each of the objectives. Do not try to clean the inside of any lenses, leave it for someone with more experience instead. Store the microscope away from dust and always use the dust cover when the microscope is not in use. This should reduce dust problems significantly. Your microscope usually has 3 or 4 different objectives.
Magnification increases as the number on the objective increases.
In order to focus the microscope you should start with the smallest objective. The microscope should provide you with a lever or knob with which you can adjust the intensity of the light. With a small magnification you need very little light to illuminate the sample. As the magnification is increased you may need to increase the light as well. Place your slide with cover-glass so that one edge of the glass is lined up in the middle of your viewing area.