That can be a bit confusing to a newbie. A brief explanation first:
Every application has a preference file. When you're in Mail or Text Edit or any application and set preferences, that creates a file that exists elsewhere to tell the Mac what your preferences are for that application. They are kept in a folder called preferences.
The path to get there is ~/Library/Preferences. When you see the ~ that means your home directory. For example, if your user name on your computer is jonfromny and the name of your hard drive is Macintosh HD, the path would be Macintosh HD/Users/jonfromny/Library/Preferences. Since you're using Lion, the Library folder is hidden so users who don't know what they're doing won't cause damage. Don't worry about that. Just follow these directions.
Be sure you're in the Finder by clicking on the desktop. Under the "Go" menu at the top, click on it. You won't see Library. Hold down the option key and Library appears. Choose it. That will open a window with your user library open. Scroll down to Preferences and double click to open it. In the search box, type "mail" without the quotes. You may see several, especially if you are using plug-ins. Some will be simple. The main pref file for mail is com.apple.mail.plist. There is also a file com.apple.mail.plist.lockfile. Delete any preferences that are obviously for Mail by selecting them and using the Command and Delete keys together.
It sounds as though you may have a corrupted preference file, which does happen from time to time. See if that works and get back to us.
By the way, when you're on the forums, if you see a path starting with ~/Library that means the library folder inside your user folder. If you see /Library, that means the Library folder inside your hard drive at the top level. When you open the, say, Macintosh HD, you'll see a "Library" folder. That is NOT your user folder, but also holds some folders of the same name, such as preferences. I know - it's confusing. The only one you need to worry about now is the one I gave you above.
