I'm studying a major illustration (nearing graduation) and all I can say is... it's been awesome. I learned an incredible amount in a mere 4 years. So if this is what you wanna do, go for it!
Good work so far, I can see art is one of you major hobbies. Your anatomy and facial/bodily proportions still need some work, as well as your digital painting (values/colours etc). But don't we all need improving?
All I can say is, draw from life, or if you're really not able to, draw from photos. lots. I see you've already been doing some studies, but keep on doing it! I can see quite a clear divide between the works where you have used reference and where you haven't. When you do studies, really study your subject. Why are certain parts placed where they are? What's the shape like? How does this differer between people?
I see you've been practising the values of skin, and that's a very good step to take! Most people do not take this into consideration, because it's such a subtle thing. So well done to you. I see you're a homestuck fan, and that's great. Just don't forget to draw other topics from time to time as well. Draw some landscapes, you'll find it'll really broaden your view (I personally started on environments too late, I kind of regret it now, because I now have a threshold to overcome to get it up to the same level as my character design). So what I'm trying to say is, draw everything, especially the stuff that's difficult for you. Be careful not to get stuck in your comfort zone.
Great start to your sketchbook You're definitely on the right track and you're stuff is looking pretty good for someone so young
All I have to say right now is just keep practising everything, but not at the same time(It's good to focus on the certain thing you want to learn). Anatomy, colors, values, perspective composition etc. are all important so those are the things you'd probably want to study.
I'll be back to check things soon, so keep updating and do so more often