Read the 12 principles of animation (ideally get a copy of The Illusion of Life, DO NOT SKIP THE BASICS!, animate bouncing balls until you’re dreaming about them, watch and study various types of animation- frame by frame if possible, and, above all else, practice, practice, practice.
A lot of people have the false impression Walt Disney animated all of his films himself by his own hand. He was an animator in the early days when he formed Silly Symphonies but as the company grew, he did less animation work and by the time they started work on Snow White, Walt had given up on drawing altogether and nothing drawn by his own hand ever made it to screen afterwards. The only time Walt ever exercised his drawing skills was when he was in production meetings working on story boards and layouts, where he would point out changes and corrections he wanted made.
There are several biographies that cover this extensively, but I think the best would be Disney Animation The Illusion of Life written by Disney's Nine Old Men of animation.
So many things. I was never a competent pixel artist because once I got to semi-proficient I looked at the skill curve and realized that I wasn't making it through all of that.
I really recommend frequenting these two sites:
http://pixelation.org http://pixeljoint.com
Someone mentioned the classic Disney animator bible:
https://www.amazon.com/Illusion-Life-Disney-Animation/dp/0786860707
It can't be understated how essential this book is to learning the essentials of animation. Further than that there were the Loomis books that were called out time and again as a great art education (palette selection becomes incredibly important in low resolution art). Those books are hard to come by-- scratch that, they used to be now they're just on Amazon.
https://www.amazon.com/dp/1845769287
There's a lot to dive in to. If all of this seems like too much, cribbing from OpenGameArt's better assets is a cheap and easy way to start.
I'll point you in the direction that I guide my students to, although these are really starting points for further discussion and education (again, I focus on why). I'll also stress that none of this is gospel, and you can ignore every rule out there. The point of learning design principals, is that you end up spending less time thinking about why you pick a certain font/color/shape, and you get straight to work. Less time = more money for you.
The genesis of motion design would be learning the fundamentals. Here are two decent (very short) videos that discuss the absolute basics (and from this, you can decide what direction of design principals you want to pursue): https://youtu.be/YqQx75OPRa0 ; https://youtu.be/udke9lMLfLQ
Don't ignore how these videos are presented to you.
For typography (my original post says typefaces, but that's just my phone screwing with me...), check out the book Designing with Type, by James Craig/Irene Scale (but make sure it's a hard copy, since the digital version replaces the fonts, and makes it nonsensical). There's no perfect book on typography, but this does a good job explaining the why.
If you only have 3 minutes to spare, watch this video (more how, less why, but it's all good): https://youtu.be/QrNi9FmdlxY
Color theory is a biggie, and there's million books on it. Probably best to Google "color theory + [insert video or animation or whatever]." This topic alone is worthy of taking a digital painting class at a community college (I stress digital, since it's more practical for video purposes).
If you only have 3 minutes to spare, watch this video (and then check out this guy's entire color theory videos): https://youtu.be/u5AnzLg1HxY
If you don't want to learn anything about color theory, go to color.adobe.com, click browse, and steal any trending palette. No thinking required (if you use any Adobe software, you can import these colors right into your software from either the browser, or the color extension).
I do focus on Weight and Distribution in my class as well, but I'm not up on current books or youtube videos. If I find some, I'll amend this post (or you can sign up for my class if you live in NE PA).
Motion Design is relatively simple (by comparison), and this (heavily ripped off) video shows 10 principles you should know: https://youtu.be/MepJ8aFDK5k (I'm aware this video is not the original author, but it's the first one that shows up on YT). These principles are all based off of 2 Disney animators (Frank and Ollie) that perfected these styles over decades. Their book is still in publication, and well worth reading.
Most books that teach motion design for After Effects will date pretty quickly, so it's best to look up decent online content, like School Of Motion.
Anyway, I hope that's a good start for you - there's a million more links I could give you, but you'll find them yourself once you get going. Just stick to videos that teach you why things are done, and forget about the rest.
Pixar in a Box: https://www.khanacademy.org/computing/pixar#start
Toniko Pantoja: https://www.youtube.com/channel/UCRTRqkhrehrY9hJJcLVUeRQ
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There are tons of videos on YouTube that can help you as well, just search the specific things you're trying to learn. If you have no idea where to start just look up the 12 principals of animation. Good luck on your journey to becoming an animator.
The Illusion of Life: Disney Animation https://www.amazon.com/dp/0786860707/ref=cm_sw_r_cp_api_i_XuLKFb2VRQTNS
Highly recommended
I tell all of my students to get this
Nothing to add on the technical side. All been said already.
Just to say: You're starting this young. As in all cases when learning a skill, this is good. It gives you time to practice. And you approach this from the exact right angle. Finding little projects for what you think might be cool, and then just making it. And then asking for feedback. Putting your stuff out to be judged by anyone can be super scary, yet here you are. That alone is praiseworthy and it's truly the best learning experience. Writers, artists, musicians do exactly that.
Just keep at it really and keep that mindset. If you do, you'll be great.
I don't know what you'll focus on, but a classic to have in mind with anything animated are the '12 Basic Principles of Animation'.
https://en.wikipedia.org/wiki/Twelve_basic_principles_of_animation
They were formulated by two Disney animators in 1981. At first glance it seems this would only apply to hand drawn animation, but it does not. It can be applied to 3D animation, video games, mograph, movies and your little floating explodey ball there. And the gun shots of you last project. They're twelve easy to understand principles that help immensely.
Or look at this short video at least.
https://www.youtube.com/watch?v=uDqjIdI4bF4
Both help immensely in understanding why and how things look realistic when moving.
Its a reference to this, although its weird that Joey Drew wrote it. https://www.amazon.com/Illusion-Life-Disney-Animation/dp/0786860707
There are two burgeoning fields that require no sexiness on paper: animation/cinema and computer programming. You can teach yourself both for free.
Just google the computer science stuff. For the animation stuff blender and The Illusion of Life from your local library. Then, many, many hours of practice (about 500 hours of deliberate practice).
...start with that. It's all in there.