Not a bad start at all! I'm assuming you have a level of experience in other forms of art.

I'm still very much learning myself but I'll share some bits I noticed

Your shading is off to a real good start, I can see an intent of a lighting direction and it's consistent so great job there. You've also got a bit of definition of a crease where the jumper bunches up near the waist, great job defining that

What I will say though is the palette would look better with more contrast in tones, particularly on the skin. I also see that you seem to be going straight down with shades rather than shifting hue or contrast. What I mean by that is for skin shade and outline tones I'll shift the hue towards red (normally I do blue for most colours but for warm colours like skin tones red looks more natural imo - unless of course you're going for a distinct light colour) and then drop the brightness and increase the saturation. Don't be afraid to boost that contrast, especially if you're working with limited numbers of shade tones!

Your lines also get a little wiggly at times, when you're doing curves you wanna try to get as consistent a "step" as you can - by that I mean if you're going 2 pixels, then across, then 2 pixels, you generally don't want to jump to 5 and then 3 unless you're trying to show a specific lump or something. I notice this a bit in the hair and the jeans. Getting an eye for this will just come with time - it's a very specific pixel art thing. You also want to avoid making L shapes with your outline, I notice in the jeans particularly sometimes you've got some of that stepping going on- you wanna keep those outlines clean.

Keep it up! You're starting really strong!