The Common Application will be available the first of July. Now is the time to check and see which of your schools uses this application.
Junior year grades should be posted by now. If you can, try and get an official copy of your transcripts from your school. When the application comes out, go ahead and start your account. Sign in and begin filling in the basic information. While this seems mundane, it is rather time consuming. Take advantage of the summer to get ahead in this process.
After completing your personal information, begin putting in your grades. Make sure you follow the instructions. One of the biggest mistakes students make is NOT reading the instructions carefully. What you think they want may be completely different from what they really want.