Hey there! First and foremost, you need to understand the form of the object you're drawing. So I'd say you need to focus on learning form and perspective. IMO, these are the two most important fundamentals you need to master. Notice that drawing straight lines kills the volume of the object because you're not following its form. Compared to shading by following the form of it. You can apply different ways of hatching/shading, but as long as it follows the form direction, it will look 3D. If you want to learn your inking, this is crucial! Look at some arstits like Antonio or Hue Teo, the way they shade their illustrations. I'd highly suggest taking the Intro to Drawing roadmap. It will help you to improve your drawing skills a lot! Hope it helps, and keep it up!
