Most college admissions offices will most likely request:
1. Your SAT or ACT scores
2. Your high school transcript
3. The formal application specific to that college
4. Letters of recommendation
5. The essay
Your entrance exam scores (SAT or ACT) need to be mailed directly from the collegeboard to the colleges you choose to apply to. You can request for your scores to be mailed to certain college right on the test, or after you take the test on the web. Your high school transcript needs to be sealed and mailed directly from your high school. You also need to fill out the application for the specific college. Most colleges will let you complete their application online, but if not, you mail the hard copy into the given school. Many colleges accept the common application, which saves a lot of time because the same application can be used for multiple schools.
Your letter or letters of recommendation should be from teachers or counselors at your school. You want to request a letter of recommendation from someone who knows your academic abilities and a bit of your personal character, and who will be sure to write nice things about you. You should always ask to see the letter of recommendation before the teacher mails it directly to the college. Make sure that the letter of recommendation is relevant to the university you are applying to (i.e. if you are applying to a music conservatory, be sure that one of your recommendation letters is from your music instructor and tells about your experience in music).
The essay is an extremely important part of the application process because it is the only part of the application where a shade of your character emerges on the page. Don't leave your essay to the last minute. Look at the topic that the college wants you to write about and give it a lot of thought. Write the essay well in advance and have it proofread by your peers and certainly an English teacher. Don't slack on this part, it really could set you apart from other candidates. You must remember that people can look mundanely similar on a white 8 1/2" by 11" sheet. The essay can show your creative flair.
Most applications are due into colleges in November or December of your senior year. If you are absolutely certain of which college you want to attend, you can apply early decision or early action, which applications are due in the early fall of your senior year. However, if you do apply early decision or early action, it may be a binding contract if you are accepted.