Hey Anon, I did a super quick paintover
I think the biggest things you should work on are:
- As other users said, the values, do some still lifes on some dramatically lit objects. Also, white backgrounds do not help painting a lot, it's better to have some kind of midtone or dark color as the base (this will help "ground" you to a midtone.
- Your colors should look like they "live in the same world" (more technically, like they are surfaces illuminated by the same color light). Warm lighting is most common for drawing people since it's the most flattering on all skin tones. The way you do this is by mixing in some light, low saturation orange with all your colors. Making things look good under super neutral white light is difficult and colored light emulates a lot of natural conditions (even sunlight, since it tends to look warm despite being white)
- Use color theory a lot, and never just use white or black when shading, remember to hue shift and to adjust the saturation respectively, hue shifting and decreasing saturation as you go darker will emulate physical paint
- And of course, keep working, do a ton of painting studies from observation and keep painting the things you like! You'll find your way around your materials quickly enough.