In case it needs to be said, these rules apply to Microsoft PowerPoint and Google Slides.
Keep a pattern. It's pleasing to your audience and allows them to quickly sort out the information. Use familiar layouts. Stick with the same fonts throughout your presentation. If you use an image effect when placing a picture, use it on all your images.
Don't use complete sentences. Imagine the words as a backup singer. Your audience should not have to stop listening to you in order to read. Most people say never to use more than 7 works in a bullet point. Things can vary a lot depending on the subject matter and the audience.
The larger your audience, the larger your font size should be.
Stay with your audience. A glance at the slides is fine. If you stare at the slides, your audience feels lonely. Do it for the people.
Classic sign of an over-enthusiastic student is a document covered with borders, text art and image effects. The very best presentations are remembered for their substance so don't let the styling get out of control.