You need to start reviewing the admissions sections at websites for the colleges to which you are considering applying. Those will tell you what you need to apply, when the application can first be provided, and when is the last date for applying. When you apply vaires among colleges but it is not in junior year in high school unless you are actually applying under special programs some colleges have that allow exceptional juniors to apply and start the fall after completing junior year in high school.
The common app, used by many colleges, comes out every year in early Aug (usually Aug 1). Colleges with their own apps will likely have varying first dates, e.g., if applying to UCs, you can apply only from Nov 1 to Nov 30. The last date to apply also varies. For regular admission, some are early in Fall of senior year, like the UCs, while others will have end dates in Jan, Feb, or even March. Moreover, many colleges have more than one application period because they have early action or early decision application periods , where you often need to apply by Nov 1 and then get a decision in December, and then also have a regular admission application period that ends oil Jan or after,