Start with soft lines until you found the shape you want, then start pushing harder with your pen.
Draw with the full arm, don´t let the elbow rest on the table.
Use good pens/markes, personally I prefer the Copic markers. They are a bit expensive but they are toatally worth it.
Practice! Work on youre technique, daily. Easy shapes, cyliders, rounds, shapes in persective, shadows. Get to know your lines and train to get them were you like.
If youre making more “serious sketches” use sketching supplies like, ellipse boards to help you get better shapes. Makes a lot for the overall expression.