Sharing use of Our new iMac

It depends on whether you need to keep your accounts separate or plan to share one account. There are pros and cons to each. Ideally each individual has their own Apple ID and you would create two user accounts. For details, visit https://support.apple.com/guide/mac-help/set-up-other-users-on-your-mac-mtusr001/mac. Then each person signs in to their own system.

Create separate user accounts for each of you in System Preferences, Users & Groups.

That will keep your iCloud account data, contacts, photos, etc. separate.