>>4073026To start, don't consciously try to improve your linework. Improve your drawing skills and that will come naturally. Good linework is 1) solid draftsmanship + understanding of your subject (whether muscles, cars, animals, anything) and 2) familiarity with your medium. The digital tablet is a medium like any traditional tool. So draw a lot generally (with pencil is good), but also practice with a tablet.
There is one thing you can do to facilitate the process: figure out what you like by trying different tools and media. I spent a year primarily drawing in sketchbooks with pencil and ink. Found out I really like brush pens and heavy, variable lineweights. That carries over to my digital practice. The problem is, the default hard round of every program is shit for that, so I'd spend an inordinate amount of time just carving, undoing, carving, undoing my lines for a mediocre result. Once I nixed that and found a calligraphy-like square brush, it was as simple as breathing. Drawing is so much better when you're working with your tools rather than fighting against them.
tldr: draw a lot and conscientiously with an aim toward improvement, know your medium, try different things