There are four kinds of college admissions: early action, early decision, regular admission, and rolling admissions.
My number one tip, before anything else, is to make sure you check deadlines for all your colleges to ensure that you do not miss your chance to submit your application. With that said, when you apply depends on the deadlines for each college and how interested you are in attending each particular college.
If you have a TOP school that you want to go to no matter what, then you should apply for the Early Decision deadline. Take note, however, that Early Decision is binding. Meaning that you agree to attend the college if you are accepted and the financial aid you are offered is sufficient. You can apply to other colleges, but once you are accepted into Early Decision at a college you must withdraw all other applications.
Early Action is similar to Early Decision, except it is not binding. Students can apply Early Action to more than one college. They will be notified early on in the year (usually in December or early January) whether they have been accepted or not.
Regular Admission is the standard application deadline, which is usually between December 1st and January 15th (some colleges have deadlines as far as February 15th). The advantage of applying Regular Admission is that the student has more time to prepare the application and gather necessary documents. However, students have less of a chance of being accepted than if they had applied Early Action, and they will not be notified of acceptance until April or May.
Rolling Admissions is not used by all colleges. Basically this is for students who are submitting their applications at the point where the college is considering them on a first come, first serve basis. Colleges with rolling admissions accept students as long as space is available.