Studying for the ACT/SAT definitely is a good starting point. I improved my score substantially between the first and second time I took the SAT, so I'd definitely recommend studying and taking those exams multiple times.
Checking out the Common App (linked below) and brainstorming answers to the Common App Essay prompts and supplemental essays that some schools require is also a good idea. I recommend preparing for any financial aid (FAFSA website linked below) and scholarship applications too.
Think about what programs might interest you and start to plan which schools you want to visit. Do research online by checking out those schools' websites and looking at YouTube/other social media sites for sponsored videos/posts that can help you get a better feel for what campus life is like. Many schools offer virtual tours and the opportunity to ask current students questions too.
I hope this helps - good luck!
Jacob recommends the following next steps: