If your family uses one Apple ID to purchase music and apps and have more than one iPhone or iPod Touch you probably won't want to share the same Apple ID account for iCloud on each phone. Doing so would sync the same contacts, inbox, bookmarks and other personal settings.
Instead, create a new Apple ID for each person using iCloud and specify in your settings which Apple ID is to be used for purchases and which to use for iCloud.
MacWorld magazine reports that Apple approves of using multiple Apple ID accounts for this purpose and also recommends configuring iMessage to use your new unique Apple ID created for iCloud.
This way you, your spouse and your teen can all share the same account for purchasing music and apps yet you each have a free 5GB iCloud account for your personal data. As long as you specify which application should use the proper Apple ID there should be no sharing of iCloud data between family users.