First, insert a slide or you specimen on the microscope stage. For first time use, get an object with a clearly identifiable pattern, like the engraving marks on a dollar bill or a business card with sharp lettering.
Use a lower-power eyepiece-objective combination. This is to avoid running the objective into the specimen and damaging both. (Your microscope may have a safety rack or limit screw to prevent stage or tube motion that will cause a stage-objective collision. Even if it does, be careful because it may not be properly adjusted.)
Familiarize yourself with the coarse and fine focus knob controls. Slowly turn the coarse focus control, noting which direction moves it towards or away from the stage and specimen.
Now look through the eyepiece and focus on your specimen with the coarse knob. When reasonably sharp, use the fine focus to improve focus.
If your microscope has parfocalized objectives, it’s now close to focus for higher powers, too. Switch objectives and refocus using only the fine focus knob. Again, be cautious of accidentally pushing the objective down into the slide.