I’m not sure if one single answer could satisfy this question…
Do you want to draw cartoons, abstraction, realism, etc.?
Different art forms require different approaches for improvement.
For example, drawing cartoons might only require constant practice and a possible reference of another cartoonist’s style, but realism (in addition to heavy amounts of practice) would require a reference of the object being drawn (no, you can’t draw realistically without a photo reference or model under appropriate lighting conditions, it simply is not possible for the human brain) along with knowledge of the different lead types required for darker shading (among other things).
In addition, realism would require some knowledge as to how light interacts with the properties and textures of objects.
For example, light could produce a certain “sheen” when applied to skin, but typically would not produce the same effect when applied to wood.
So, depending on how you’d like to draw, you may find that you’d not only need practice to improve your skills, but you’d also need to study in order to improve the overall quality of your art.
Practice a bunch, practice everyday, practice whenever you get the chance to practice.
Books, YouTube, etc. can definitely help you, but deriving skills from observation can be difficult without natural talent (or acquired expertise); a good teacher could take you very far.
It’s sorta like being a good gentleman; get a general idea of the history of art and the various artists, their techniques, etc. and take what you can learn from those artists.
Also, make sure to show your art to other artists; don’t just hide it away.
Your peers and superiors can help you improve your work by pointing out the flaws in your art.
(Make sure their criticism is actually helpful, though; if not, punch 'em in the face.)