Step one is having a tablet that isn't way too small. With those tablets the small surface area is akin to having the sensitivity on your mouse cranked very high.
Step two is to do the exercises in these videos like hell...
https://www.youtube.com/watch?v=wgDNDOKnArk
...to get good at freehand drawing, however with that said, a lot of digital lineart is done using "vectors" which is a process I don't know too much about myself, but I do know you don't need to be proficient in drawing straight lines, curves or ellipses to make use of them.