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I'd recommend to use one Apple ID on all devices. Creating multiple ID may create problems in syncing devices, downloading etc. If you forget details of existing IDs then you can reset them.
To create new Apple ID, you will need a new email address or the one you have not used to register with Apple before. You can create a new Apple ID from here --> https://appleid.apple.com/account
Once you created a new Apple ID, then add it to your iPhone. Go to Settings > iTunes & App Store. Tap old ID and tap Sign Out. Then sign in to new Apple ID.
If you would like to use this ID on iCloud as well, then go to Settings > iCloud and sign out from that account. Keep files if asked. Then tap Sign In and sign in to new account.
Note : When you try to update apps and if your iPhone is asking for old Apple ID, then you have to delete those apps requiring update and you will have to redownload them using new ID.
If you have any difficulty on following the steps above, please let me know and I will continue to help you