Please do not take the following criticisms as an attack .
Do not start off working on a tablet. Always start your artistic studies with traditional media until you get the hang of the physical form enough to translate it into a digital format.
If you want to continue working on humans, humanoids, or variations there of you need a STRONG understanding of anatomy. If you are young I suggest looking at photos with people in "nude suites" and if you're older I suggest Classic Human Anatomy by Valerie Winslow and also Andrew Loomis (all his books). If you have access to a human skeleton (like your science lab) then use it.
For perspective and environments, there are many tutorials on deviantart and looking at your own surroundings always makes a difference.
Color is something I still have trouble with so I'm not going there.
Most importantly, practice. Practice, practice, practice. Because, honestly, that's the only method of getting better.