This is a very big and difficult question, and one I suspect have different answers for different people. But, here's some of my thoughts:

Making stuff up is a skill, and like every other skill it needs training. The best way to train is usually not just jumping directly into the deep end of the pool, and you certainly shouldn't expect yourself to be good at it right away.

One good way train is to start with something "generic". Pick a thing you like, let's say for this example Dragons, and sit down and draw one. Don't draw a specific one, just "a dragon", and don't worry too much about making the design elaborate or unique. In the process, you'll probably start noticing all the decisions you have to make. Does it have horns? In that case, what shape are they? Just pick an answer and go for now, but one you're done the first one, try making a second one and explicitly go for different answers to these questions. This gets you in the habit of thinking more about design, and helps you form your own opinions and preferences.

Another good exercise is to join some kind of art challenge with a prompt list, like Inktober or similar. With a set deadline and a prompt to start from rather than the nebulous "whatever I want", you can get into the habit of quickly forming and trying out ideas.

And whatever you do, don't be discouraged if your first attempts at original stuff comes out bad! You'll get better!