Also, letters of recommendation from teachers can be really important in this process. Take some time to build relationships with your teachers, if you haven’t already, by speaking up a lot in class and chatting with them during breaks so they get to know you better. And when you ask for recommendations, have a conversation with them about it- ask if they feel like they would be able to write you a strong letter and what is information you could share with them about your accomplishments and goals that would help them write it. I know some students find this challenging and feel like they don’t have a lot of options in who to ask, but it’s definitely a red flag when admissions folks see a letter that is lukewarm, or even explicitly brings up some negatives. Talking with your teachers ahead of time should help you vet whether this is someone you want to be asking to speak on your behalf!
However, you know what works best for you. Do you enjoy learning in a classroom setting? Are you able to retain information from lectures? Is your preferred learning more hands on? Do you already know what career path you want to go down? Think through these things critically and do as much research as possible before applying.
Once you've chosen your colleges to apply for, Kali nailed it. Take time to understand what's being asked and don't rush through it. That said, don't delay/procrastinate so you have to cram in so many essays, etc.
There is a website - myintuition.org - that allows you to input your data and get an idea of what it would cost you to attend a range of well known schools. Once you start looking at specific schools, don't forget to input your data into their net cost calculators to see how much they expect you to pay and what kind of scholarships you might be receiving. When you are finalizing your list, make sure you have several financial safety schools, meaning, you know for sure that if you are admitted there, you'll be able to afford it.
I think one of the common mistakes I made was worried about where I thought I should be and where I was expected to go, rather than actually thinking critically about what I wanted. I think a smart thing to do would be make a list of majors, coursework, clubs, and other important characteristics that you think matter to you in a school (ie. university size, class size, location city vs suburbs, test scores) and then make a realistic list about what schools you want to target. I would also highly suggest going to visit campuses and taking informational tours. Try to image what you would feel like living on that campus for 4 years.
Hope this helps!