Make sure to look for stabilization options in your drawing program, it will help your lines from being too shaky. As far as learning how to draw with a tablet, I started out by drawing the sketch on paper, scanning it, and doing line art on the computer. It was the best way to transition, since my sketches would come out much better on paper at the time. Before you even realize it, you'll be able to do start to finish on the tablet, you just have to keep using it.
Also, I don't know what what your reasoning for using free drawing programs is, but I would honestly recommend getting Clip Studio Paint or Paint Tool Sai. CSP tends to go on sale all the time, and the basic version of the program averagely goes down to 25 USD. Using a program that has a lot of users can enable you to find the proper help and tutorials in your journey to learning digital art.