Currently Being ModeratedFeb 6, 2013 8:54 AM (in response to gregoryfromchicago)
It's based on your Apple ID that was used when it was set up. Go to Settings>iCloud and sign in with the ID for the iCloud account you want to use. If you aready have a different iCloud account set up on the phone you have to go to Settings>iCloud, tap Delete Account, choose Keep on My iPhone when prompted, then sign in with the correct ID, turn on your iCloud data syncing, and when prompted choose Merge to merge the data on your phone with the data in the existing iCloud account.
Note: sharing an iCloud account does have disadvantages. You will be sharing the same 5GB of free iCloud storage for your backups. Also, any data that you sync with the shared account (contacts, calendars, etc.) will be merged and the merged data will appear on both devices (so you may, for example, end up with each other's contacts on your phones). You will also be sharing the same iCloud email inbox. If you don't want this, then you should keep separate iCloud accounts on your devices.