It varies based on the school, but you have to apply early decision, check the box to do so, and get it in before the ED deadline. Some schools have a thing like early decision, but it is binding. That is, if they let you in, you agree to withdraw from all other schools and put down your deposit at the early school. This is the proof that they are your first choice and thus deserving of a boost. Other schools will give you the boost just for applying early and you can apply to more than one school early. Read the fine print since it varies.
If you are borderline and they don't want to admit you early decision, most schools notify you and say they will then consider you with the regular applications.