Hey Dero!! Ahhh thanks so much for your comment on my gallery-- and I'm so sorry this comment is coming so late, I really wanted to give a more meaningful comment since I'm super excited that you're getting into drawing! Your sketches look great so far, but let throw in some tips that'll hopefully help you out a little! I'm by no means knowledgable on a lot of technical details in drawing, so take what I say with a grain of salt-- and anyone who just wants to bust my ass for incorrect advice, please feel free!
Jumping right in, to add to what Alexial said about working on hair, you should consider the weight of your hair. Not like the physical weight, but the way you balance your detailed strands on the inside, versus the strands of the outline. If you look particularly at the second bust of the girl you posted, you'll notice that the strands of her hair that you shaded in are quite fine-- but the overall shape of it, the darker outlines, are quite thick! The contrast can be a bit disorienting, so I would suggest choosing either going all out with the finer hair details-- or going full chunky. (But remember, if you go full chunky, there will still be 'sections' of hair, so you can't leave the inside blank like it is on the first sketch with the outstretched hand!)
I hope you don't mind if I redlined a bit to show you what I mean-- but on this particular work, I just had three particular places I wanted to point out:
1. I think the fat on the arm would actually push this line so that her forearm appears closer than her upper arm. It's just the positioning of the line to change which side of the elbow the fat appears on, but it's quite important!
2. Her whole body is a bit on the short side! It can be quite interesting, trying to proportion a whole person onto a limited space/piece of paper, but better to sacrifice drawing the knees than to squish her down! This tendency to shorten the torso also appeared in the first sketch with the outstretched hand, so I recommend you pay close attention to how long the torso is in relation to the head!
3. Finally, I highly encourage you not to hide the hands! Especially if you're practicing them-- Hiding an arm behind the back like this ends up in an awkward pose, and a missed opportunity to practice the hand-on-hip pose, which is quite common I find!
There's always stuff to improve on, but I seriously commend you on quite a lot of things that I never realized for the longest time when I started drawing that you seem to have picked right up! I'm super impressed that you've paid attention to where hair parts from out of the head, which is something I straight up ignored and didn't understand until just a year or two ago!! It really adds to a piece in my opinion, so great job on that-- and I'm also impressed by your eye for details most people wouldn't even think to consider at first, such as collarbones and defining the location of the knees.
Great job, I can't wait to see more! *V*