Some of the foundational art skills you can demonstrate in your portfolio are: still life drawings, color and value studies (to demonstrate your handle of light + shadow + color theory), and figure drawing. Keep in mind, your portfolio does not need to be full of completed masterpieces - including pages from your sketchbook that shows 5-minute exercises can be valuable additions. Admissions staff want to see that you understand these concepts and are eager to learn them, even if you're not perfect at them yet (that's what school is for, after all!).
Specifically for animation, your portfolio can include: storyboards that are simple yet clearly tell the mood or narrative you want to express, facial expression studies, movement studies, concept art, character design, etc. You can find a lot of references on Pinterest or simple tutorials on Youtube to start building these pieces.
Congratulations on you graduation!
Addi recommends the following next steps:
Each school may have their own requirements to get into a program. Research the schools you're interested in, and see what they require. Some will ask for portfolios and you choose what to include, others will be more specific, for example provide a website showing your 10 favorite pieces of art. Some schools have you start with basics (and will teach you skills) and then apply into an advanced program using your portfolio from those entry classes.
Keep practicing your work and staying current in the methods/technologies you're using. Experiment with different styles. Research the programs you're interested in to learn about requirements. Then, build up your expertise and portfolio based on those requirements.
Naomi María’s Answer
Check out art schools near you online. You'll be able to see student work, see what art media the schools focus on: painting, printmaking, design, illustration, sculpture, new media, ceramics, or general fine arts. One thing to note is some schools may not say that they have an animation program, but it may be part of a different program like motion graphics, new media, or something else. Feel free to email the art school with questions and get more information.
While you're looking into your options, I recommend having a physical sketchbook to try drawing in at least a couple times a week. If you do digital illustration, painting, or other art beyond your sketchbook, it would be good to create a portfolio of your best work. It's easy to create an online portfolio with Tumblr using a clean and simple theme. Also think about what you like to make and why you like to make it, which artists inspire you, and what kinds of art or animation you'd like to create. These things are what you'd be doing regularly in art school, so it helps to start thinking like an art student!