Most high schoolers actually start applying to colleges at the start of their senior year. On this post you'll find a super brief timeline of the college application process that will help you advance through it as stress-free as possible.
Summer before Senior Year
If you're reading this and you haven't started your senior year yet, you're on a great track. Use your summer before senior year to find out how many colleges you should apply to. Your goal is to know exactly which colleges you are applying to by the time school starts. Narrow down or increase your number of options, some ways to do this are visiting colleges in-person (if possible), talking to students from there, or simply doing research on the internet to find out if a college is right for you. If you are considering asking teachers/employers for letters of recommendation, ask them during this time, as it gives them plenty of time to write a solid letter for you before any deadlines. Having all this figured out will prevent you additional stress in an already busy start to the school year.
This is when you should actually start applying to your colleges. The very first thing you should do is find out all of your colleges' application deadlines. You may be surprised to see some deadlines are sooner in the fall than you expect, this is especially the case if you are applying early decision/action. The last thing you should do is procrastinate your applications (and definitely never submit any the night before they are due). Ideally you should be done with each application a few weeks before they are due, and finally submit them no later than a week before. If applicable, also start and complete any financial aid applications and send ACT/SAT scores during this time.