It's a great idea to get started on the Common App. http://www.commonapp.org/how-apply
If you don't yet have an account, create one today. A few minutes of your time will allow you to enter your basic info, and add the colleges that have risen to the top of the College List we've prepared. Not that you can't add more -- this is just the preliminary step and colleges may be added and taken off as you get the hang of the site and as you refine your plans over the coming months.
Although over 600 schools use the Common App, you may find a few schools on your list that do not. MIT, for example, has it's own unique application at https://my.mit.edu/uaweb/login.htm
But wait, there's more! Most schools that accept the Common App also accept the Coalition App http://www.coalitionforcollegeaccess.org