General
START EARLY!!
College apps are essentially another extracurricular activity. Most people take a long time to write and edit essays, so do a little bit of planning/writing a couple times every week. You will be busier than you think, especially if you apply to a lot of colleges. You will also get burnt-out (that's ok; it's normal), so prepare for that and don't wait until the last minute to finish off your applications.
Make everyone read your essays
Be "pointy" & don't be boring
Universities want well-rounded classes, not necessarily well-rounded students. Show that you are interested in a subject or your major with proof (internships, clubs, leadership, research, etc.). Have something to say about why you like what you're studying, and say it in a way that draws attention to you.
Reuse your essays
Many colleges have similar themed essays ("why major", "diversity", etc.), so save yourself some time and copy and paste. Just make sure you don't have the name of one college in the application essay of another college.
Tour locally
UT is HUGE, Rice is medium, and Trinity is small. Get a sense of what size colleges you like. Do you like big football games? Do you need more one-one-one time with professors and TAs? Visiting different schools can help you decide what you like.
Resume and brag sheet
These are forms to give to teachers and counselors so that they can write you rec letters. You'll include information about yourself, your intended major, clubs, etc. to give them a better idea of who you are as a person in non-academic settings.
See here for Carnegie's brag sheet template (required by counselors but not by all teachers). Look online for resume ideas.
Spreadsheets are your best friends
Other factors to consider: campus size, surrounding area (small/big city), transportation (bike, walk, shuttle, car), weather, housing guaranteed/required, study abroad, quarter/semester system, research opportunities, internship access, access to ethnic enclaves (ex. Chinatown)
Common Data Set
Search up "[college name] common data set", and you'll find a gold mine of information that universities publish every year! They tend to include information on student enrollment numbers, what factors the university deems important on freshmen applications, SAT/ACT score ranges for admitted students, number of bachelor degrees conferred for each major, etc. It's a lot to go through, and it's not necessary, but it's available if you know where to look.
Ex. this is the common data set 2021-2022 for UMich
Don’t apply to a college if you can’t make a “why college” essay.
Top 6% get automatic admission to UT Austin but not necessarily to your first choice major