It’s hard to make a good meal without good ingredients. Visual design is the same. You should use high quality design materials.
You might make some from scratch. Like your colour palette. It’s up to you to learn how to make a good colour palette.
But some design materials are provided by other people. Typefaces, illustrations, and icons are three examples. Materials like these should be high quality.
Keep in mind that high quality ingredients won’t make you a good chef. Good design isn’t easy just because you have a good typeface. But it’s harder if you don’t.
You have two choices if you want to use high quality design materials.
The first is to learn what high quality means for each type of material. The second is to find other people with good taste and use what they use. The second way is much easier.
The second way will also teach you more about the first over time. The more you look at what other people consider high quality, the more you will understand yourself.