This is slightly different for each computer application that you use. Generally, you must first open your presentation using the presentation software that created it. Most of these programs only require a mouse click or a tap of the space bar to advance one step.
Steps are either the addition of a bullet to a slide, or the flipping of a slide.
Often, you can go back one step by either using the backspace or left arrow key. You should step through the slide show as you are speaking.