First you need knowledge about all the blocks you can use.
You need to know how they work too. For example, stairs can be upside down, trapdoors can be opened and closed, etc.
You can either use your imagination or take inspiration from anything else.
Minecraft, another game, real life, etc.
Don't stick to one material and don't just make a square.
Use variation with your blocks and and shapes.
Also, everyone prefers different colors but you got to learn how to use them. For example, I like green, purple and orange, yet I still can't use them at the same time because it doesn't fit with my builds or it will look too busy.
White, gray, black and brown are very useful colors as they can be mixed with just about anything.
So that would be stone, wooden and dyed blocks.
And one last thing: Don't make builds too empty or busy.
Decorate empty spaces and remove unnecessary stuff in busy spots.
What I recommend doing is practicing.
Keep making builds. But you can change them if you don't like it.