You can't. You cannot change your current AppleID which you are using for iTunes store purchases (lets say its a gmail account) to a @me.com account.
But what you can do and most of us have done is keep that AppleID for iTunes purchases. When you install iOS 5 and go through initial setup, when it ask for AppleID for iCloud, say create new one and it will let you create a free MobileMe account. If you have already set up device using your iTunes AppleID, go under Settings>iCloud. Delete the account. Then go into Mail settings and add an iCloud account. At the bottom, it will say create AppleID. Follow the steps and it will give you option to create a free @me address.