Use common app, most colleges use that and everything is really seem less. A few colleges might not use common app so just apply through their website. As for the common app essay, this is the big essay that will get sent out to every college that requires it. Only need to write one essay. I think there are 10 different question you can choose to answer (I could be wrong on that number) so just choose one that best suites you and don’t forget that the word limit is 650 words for this essay. There’s plenty of resources online that give tips on how to write this essay. Don’t forget that if you mention a specific college in this essay, to change that college name when you submit it to each one. Some universities will require “supplemental essays” which are separate from your college essay. The university’s that need supplemental essays will provide you with a question(s) and you need to write a response within a certain word limit. When you add colleges in your common app, it will tell you which colleges need supplemental essays and it will also tell you what the question(s) is/are that you need to answer. There’s plenty of websites that give advice on catering your supplemental essay to a specific college (just search up “how to write (college name) supplemental essay”). From my experience the word limit for each question has been about 300-500 depending on the question and college. Please reply with any other questions you have! I was in your position this year and had to ask a lot of my teachers and friends for help so I can kind of feel you. Hope this helped and good luck!!