The answer is simple: as soon as you can! Whether it be from your freshman year to your senior year, we recommend that you begin the process of figuring out what you want your resume and academic record to look like well before you actually start filling out the application.
We recommend that you pinpoint certain extracurriculars, internships, and volunteer opportunities you want to commit to early on so that you can build a credible record for college applications. The key is to show your commitment and dedication to whatever activity you chose to be a part of.
As far as academic achievements, depending on your ability and comfort, try to take the most challenging classes available to you. Whatever class you take, whether it be AP, Honors, or regular, you should try and achieve the highest grades you can. Planning is key to having a great college application, so the earlier you start, the better your chances will be.