I'll give the most basic art advice ever. Bear in mind it's 5% insulting.
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1. Circle Tool. I see that with the head shape. Learn how to make chin structures, jawlines, facial sizes.
2. No hands. I see this with beginner artists, so learn how to sketch joints, especially palms and fingers.
3. Too thin. The body looks like a stick figure. Lean weight differences and how to draw each one.
4. Lack of composition. It's basic colors, so mess with the color tool. Add shading and highlighting.
5. Eyes. There's no pupils, so for now add black dots, but soon, learn how to draw eyes.
6. Face. There's no nose or jaw, or lip. Learn how to draw faces, again. Details are important.
7. Size differences. It's too wonky, so either work on similarities or use to copy paste tool.
8. Blackening the lines. Either don't do it, or do it in all parts of the body.
I'd rate your art, mediocre for a beginner. I've seen worse, but at least you admit to your slight mistakes. Those are easy to fix, so you're good.