i have two phones usisng same apple id. how do I remove one and change to Family Sharing?

Your husband will need an Apple ID:

Create an Apple ID - https://support.apple.com/HT204316

Ideally do this so everything is initialized for him on the new device:

What to do before selling or giving away your iPhone, iPad, or iPod touch [or if you received one that had not been erased] - "Use these steps to remove your personal information from a device, even if you don't have it anymore." - https://support.apple.com/HT201351

Read these two links to get started with Family Sharing:

Set up Family Sharing - https://support.apple.com/HT201088

Sharing purchased content with Family Sharing - http://support.apple.com/HT201085

Erasing the device is going by-the-book but may not be 100% necessary or even desirable if you are wanting to keep some common things on the device. I have known couples who want to have some things jointly on a device and other things individual. They had both phones set up under the same ID for most things but had a separate AppleID for Contacts and Photos, Mail. This was customized under:

Change your iCloud feature settings [iOS and Mac] - https://support.apple.com/HT207689 - "'When you turn off an iCloud feature on a device, information stored only in iCloud for that feature is no longer available on the device. If you turn off Mail, Contacts, Calendars, Notes, Reminders, or iCloud Photos on just one device, you can still view and change your information on iCloud.com and on another device you set up for that feature."

It's a bit more complicated to do but it depends upon your exact desired configuration.

Another way to do it is to not actually erase the entire device but instead to change the Apple ID on the device and then go through each Settings item and configure that. For example, if you change your iCloud settings for Contacts it will ask if you want to delete old Contacts before syncing to the new account. If you have been sharing contacts you may not want to have to type them all in again and will want to tell it to keep old ones on the phone when changing the Apple ID on contacts. Then your husband will have all the old contacts and can delete the ones specific to you and not have to type in all the old contacts or have you Airdrop them all.

If you do not erase the device then old apps will also still be on it. You will want/need to use Family Sharing to update these but if you have one that has disappeared from the App Store then you may wish to not have to redownload all apps as part of completely erasing the phone.