I recently got my tablet, and I attempted some more fanart, specifically of Mahir in a specific scene. As you can see, my lines are still incredibly unstable. Also, yes, Mahir is merely a floating head in this one, as I find bodies and clothing still somewhat difficult to draw.
Oh hey, here I am replying to your other message and you've already started! Nice! Don't worry about the wobbliness, that's something everyone struggles with at first, on any media, and tablets are especially bad because they're like trying to draw on glass. I actually tape a sheet of paper to mine (just painters tape, so it doesn't leave a sticky mess) so I can get some friction. There are also smoothing/stability settings in a lot of software, but beware of cranking those too high: you lose the ability to make fine motions you WANT to make. It's like how people with botox are so tightened up they can't frown anymore. TLDR: a little wobble is due to tablet/software, and you should tweak the settings to try to get rid of that, but the rest is due to hand, and will fade with practice. A good way to test is doing quick, sweeping arc lines or "straight" lines, just a really quick motion across the tablet. If the line is smooth in the art program, all is good. If it's still all over the place, that's jitter, and you should try to adjust settings until that goes away.
I'm sure you'll figure this out on your own too, but if you aren't already, put your lineart on a layer above your colors; that way you can color INTO the lineart, but you'll be beneath it, so you'll get a nice clean meet of line and color without the little gaps, and you won't have to worry about super precise or overrunning your lines. I kinda dig the rough style this one has though.
And floating heads are great! That's what I started with for a long time. They're simple enough to not be frustrating, but there's enough there to have fun with, and even to practice turning stuff in 3d, getting volume and depth, all kinds of stuff. They're a good starting point.