What is your direction with this sketch? Is it going to be a concept-art ish character-on-white-background kind? Or is it going to be an illustration with backgrounds and stuff? If it's the latter, then the composition is lacking (no foreground, no background, bad placement - see rule of third).
Anatomy is okay, although the head is too big. Left leg is a bit too stiff (stretching the leg 100% straight is actually uncomfortable - although it could look like that because her pants aren't tight), left feet should tilt away from the character a bit.
The tree is totally out of place. It's too small for something with a bunch of scars and rings on it. Also you should grasp the basic concept of a tree. It's not a fork stabbed into some round furry objects. If you inspect real trees, they have numerous tiny branches - draw those tiny branches first to define the shape of the tree, then add in leaves later.
Well to be honest I had none when I started work on it, I drew the girl first, than added in the tree afterwards as an afterthought.
Still, I have taken your adivice to heart and have begun to focus on drawing smaller heads on my characters, as for the tree as you mentioned it doesn't really look like a tree, but I added in the scars and rings to at least give it a bit of dept, but other than that I had no real knownledge on how one draws tree since I lacked a decent ref at the time.
I see, there's nothing bad about doodling around without directions, so I won't blame you for that. My advice would be to scrap out the tree completely. With how big the character is and how she's positioned, there's pretty much no way to fit in a tree (you can fit in a big trunk, but composition-wise, it still looks weird). I suggest putting a large rock there instead (without making the rock vertically taller than the paper). Something like, the character sitting in a grassfield, leaning her back to a big rock.
That's much better. About the little branches sprouting from the ground, you don't need to add rings to them. Also they should be a bit thinner.
If you want to take this into a full background drawing, then you should fix the composition a bit. Look up "rule of thirds" ([link] and [link]). You'd want to line up your character's head with that top-right intersection. It makes the image flow much smoother. Although I dunno if you could be bothered to draw it again cause it's a traditional work.
Starting over from the start would be the best idea to avoid messing up the paper even more, I’ll see if I can find time for it this upcoming week.
Rule of thirds... This is why I wish daily that my school had an actual art class rather than dumping it on an idiot teacher whose idea of teaching art consisted of "Sit still, don't speak to anyone and listen to me babble on about stuff not related to art before playing with some clay."