During your Senior year you will submit applications to perhaps 6-8 universities. Most private colleges use the Common Application, where you complete one application and send it to many schools at once. Most state universities use their own applications, which you complete on their website. For all of these applications you can save your work and log out, and submit the application another day. Most students work on the applications for several months before submitting. The application is your *one* chance to convince the admissions committee that you belong at their school. It doesn't matter what you've done for four years, if your application doesn't show it, or if your application has errors, your admissions chances are decreased. Proofread your apps carefully and ask at least one other person to look over them as well. Put your best foot forward!
During the summer before your Senior year you should write as many of your admissions essays as you can. The Common Application goes live on August 1st, so you should have that fully completed before school starts. Some colleges will start to post their new applications during July and August; nearly all colleges will have their new applications live during September. Beware: before you begin any application, always check the dates very carefully to be sure you are not working on last year's application, or you will lose all of your work when the new app goes live.
Plan to submit your applications September through November, and try to be finished before Thanksgiving. Watch deadlines carefully, and consider applying for Early Action or Early Decision. Don't worry; CollegeMapper will guide you through!