For starters, the obvious thing to do right away is clear out anything you don't need to store in your email. Especially emails with pictures and attachments. Try to get in the habit of either saving them to your computer or deleting them immediately. Also go through and clean up your contact list, (do you really need your son's teacher's email and all of their info from 4 years ago?) Another thing that may be overlooked is your sent folder. (I am bad about that myself) Get rid of everything you don't really need.
Also, go through and clean out old calendar reminders and events that are no longer needed or obsolete. Try to clean out as much as you can. After you have done all of that, let us know if that helped.