Charles Bargue's Drawing Course ("cours de dessin" - it's a book) is a classic from the 1800s. It's also in the public domain!

https://archive.org/details/C.BargueDrawingCourse/page/n23/m...

The idea is to slowly progress through the book by copying each plate accurately (the sizes are scaled down in book form, so zoom in!). I found it very effective for practicing lines, tones, and form. It's nice to pick up fundamentals first before delving deeper into a specific style.

And the #1 most important thing is to just draw as much as you can. There are tons of before-and-afters on social media where people with awful beginner drawings have shown that they can improve to become very good artists by simply practicing a lot. Much of drawing is raw mechanical precision - and it can and must be trained like any other physical skill.