Before you can actually start your applications, you have to choose which colleges you'd like to attend. There are many different types of four-year colleges, and the ones you choose should match your interests. Liberal arts schools are smaller than state universities and teach a broader curriculum than art colleges, for instance. Once you know where you'd like to apply, decide whether you'd like to apply early or regular admissions and use the application timeline relevant to your status to make sure that you stay on track.
Four-year colleges come in all shapes and sizes, but all provide students with an immersive living and learning environment. Research universities are larger than liberal arts colleges and offer more degree tracks. Art colleges and conservatories are specialized institutions catering to the visual and performing arts, though research universities and liberal arts colleges also offer perfectly respectable arts programs.
For most colleges, there's not just one application deadline. Students who have a dream school in mind may choose to apply early decision. Students who are pretty sure they want to attend a certain school but want to keep their options open can apply early action. Others may elect to apply regular decision, leaving more time to complete the application. The choice is entirely up to you.
You can and should start thinking about how to make yourself a competitive college applicant as early as your freshman year of high school, but the most work doesn't come your way until junior and senior years. Use the time before then to apply for private scholarships and discover where you want to apply to school. Then, you can focus on narrowing down your list and actually submitting applications when the time comes.
As a returning or adult student, you are not subject to the same rigorous timeline as high school students. The process of applying to college as an adult can take about a year from the time you've made the decision to apply and actually starting school. On your to-do list are talking to your current employer and family about your decision, finding schools that fit your needs, taking any necessary standardized tests, and actually completing your applications.
As a transfer student, you need to apply to a new college while simultaneously completing your current classes and keeping your grades up. As soon as you know you want to transfer, you need to find prospective institutions and start working with an advisor to learn more about transfer applications and credit transfer. Since you recently completed the application process for your current school, know that you'll have a lot on your plate until everything is submitted.