Behind Good Design
I enjoy when things look good and work the way it intended. I'm not a designer, so when it comes to a lot of design questions, e.g. color, I will not point you to a mighty Color Wheel, but would rather recommend my personal approach: google "good color pallete" and pick what you like from there.
There are hundreds of "Hundred tips how to make your presentation look better". In this specific area, I believe it's important to avoid memorizing tricks and to understand principles behind it.
Attention path - Understand it
Befor you can judge any design, you need to learn how people see things. There is a visual hierarchy everywhere. Our eyes never follow the simple line, but rather jump from one object of visual importance to another, guided by size, color, complexity, whitespace and even some ancient instincts, e.g. looking at a human face first cause it could mean danger.
There are couple of basic rules: images usually more complex so get more attention, bigger text reads first vs small text, Z and F patterns of the eye path. There are also a lot of great articles on the scinece behind it and the way to control attention. Start here.
Contrast - Create the order
Contrast is your first tool to create the order from a chaos. Bigger object are more important, smallealways go second. It's not just about the size, but rather visual weight. Place your content on a slide. Than add contrast by highlighting headers and subheaders. Use contrast to structure your story and control the way people will read it. Stay aware. Too much contrast can distance things as far as being from a different stories. Too little could be confusing and percived simply as a sloppy mistake.
Proximity - Control the flow
Whitespace separates objects, but the right use of it could as well connect it. Proximity of two blocks of information creates connection between them, guiding viewers attention. Place header in the middle of two paragraphes. Now move it down, sticking to the text it belongs too. Can you see how easier it become to reade it? Give content a space to breathe by surrounding it with a whitespace. Connect related blocks by placing it closer. Break the relations of unrelated blocks by placing it obviously further.
Consistency - Polish the notches
Whatever your personal design believe, stick to it. If you used something once, use it everywhere in a similar context. Avoid text jumping in position from one slide to another. Choose if your bullet points style. Stick to one capitalization principle. Do it similar, cause this is the best way to make things look clean and nice.
Once you understand rules, you can start breaking it. By breaking the rules you will understand it better.