So if I understand this, you will be making your own account. One of you will set up Family Sharing and be the Organizer. It can be either of you.
When you sign out of the old account you will be asked if you wish to delete the content currently on the iPhone. Say no. You want to keep your content. Read what this does --> What happens after you choose Sign Out But Don't Erase - Apple Support
If you were using Photo Optimization, turn this off so full quality photos are kept on the iPhone --> Manage your photo and video storage - Apple Support
Also make sure you are signed out of Find My with the old account --> Turn off Find My on your devices and items - Apple Support
Read this document to learn how to sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your devices - Apple Support
For an Apple mobile device:
1. Go to Settings, then tap your name.
2. Scroll down to the bottom.
3. Tap Sign Out.
4. Enter your Apple ID password and tap Turn Off. (Read this if you If you forgot your Apple Account password - Apple Support )
5. Turn on the data that you want to keep a copy of on your device.
6. Tap Sign Out.
7. Tap Sign Out again to confirm that you want to sign out of iCloud.
You can then create a new Apple Account for yourself --> How to create a new Apple Account - Apple Support
To learn how to set up Family Sharing, read this support article --> How to set up Family Sharing on your iPhone, iPad, or Mac - Apple Support
Note that despite doing all the above you may still lose some content if it was specifically associated with that old account. Some data are Apple Account specific and will not transfer.
If you wish for somebody at an Apple Store to help you, set up a Genius Bar appointment. Go to this page to find your nearest Apple Authorized Service Provider (AASP) or Apple Authorized Distributor (AAD) --> Find Locations
How to make a Genius Bar appointment
1. Go to Genius Bar Reservation and Apple Support Options - Apple
2. Go to "Get hardware help".
3. Select your hardware and continue selecting options requiring replacing parts until you see "Bring in for Repair"
4. Sign in with your Apple ID
or:
Refer to this document for ways to contact Apple ➞ Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone ➞ Contact Apple Support - Apple Support
