This quick 10 minutes guideline will explain basic principles of designing good deck.
This is one single and most important principle, and if you follow just this, it will greatly improve chances of success.
Human brain has a very limited processing power and unlike cyborg's brain, was never designed for processing complex amounts of data. It can process no more than 5-7 elements of data at one given moment, and 3 is the magic number that works always.
Here's core aspects of simplification:
The most common mistake is to try to fit in one slide what normally should be in separate slides. If your slide starts feeling too crowded, it make sense to separate it into several slides. Think of it as a food: you don't try to eat the whole meal at once, you do it step by step.
From the design perspective, the less elements you have on page, the easier it gets to design and organize them. Designing slides with a lot of elements can be a challenging task even for professional designers, because it requires setting a balance and proper relationships between all of them.
Cutting everything non essential, and separating large pieces of data into separate slides will help you to achieve clean, readable designs - without requiring any advanced design skills.
Be a samurai. Cut, cut, cut. Cutting, reducing, and deleting everything that is not absolutely necessary is the key designer skill. That includes: large sentences, long headers, all unnecessary elements.