What are your goals as an artist? This is the most important starting point. From what you've described, it sounds like you want to learn to draw from imagination. Even if you want to draw from imagination, though, you should do some observational drawing. It will train your eye and help develop your mechanical skills with your pencil, leading to better line quality and control.

Learning to draw from imagination is hard. It will take a lot of discipline and drawing to get to a point where things look good - take this as an opportunity to go wild and draw whatever you want. It's going to look bad, but it will be a huge confidence booster later on when you look back at some of the stuff you drew (as well as a great source of redraw inspiration!). There's a lot of resources around the internet regarding learning how to draw from imagination, but the important thing to keep in mind is that drawing from imagination and drawing from observation are two different beasts altogether. If you want to learn how to draw from imagination, you need to study what artists call the fundamentals - perspective, anatomy, form, values & lighting, composition. They're hard. They can be boring. And you absolutely need to learn them if you want to improve.

You will likely want to start your studies with perspective and form. Drawabox and its exercises are great for developing these two fundamental skills. If you really want to master perspective, though, I highly recommend Scott Robertson's book, How to Draw (Though Drawabox also has a lot of the information from that book). Proko is a great free resource to study anatomy. Look at both his figure drawing and anatomy for artists playlists - they're both incredibly important if you want to improve. Look into New Masters Academy as well - they have free life modelling session videos on Youtube you can draw from for study. Learning values and lighting and composition is also important. You should create some still life compositions and draw them. Not only will this train your observational skills, it will improve your understanding of what works and what doesn't.

There are no shortcuts in drawing. Some people improve quickly and some people improve slowly. If you want to improve, draw. In that regard, it's the most honest skill in the world.