Apple ID linked to Another

I have my complete own Apple ID. My computer has always been connected to my dad and mine Apple IDs (I’m not really sure how ??) because we shared it. But I just recently got a new computer, and when I log on with my email and password, it signed me in as my dad and synced all of his iCloud info (contacts, images), but it synced my messages and call logs. It doesn’t make any sense. I want this computer completely not associated with his Apple ID. I logged out of his and into mine again and this time it said it was mine with my email and name instead of his, but the profile picture and phone number were his. So random. Under his Apple ID, it makes all of his devices and then my computer and Apple Watch. How do I disconnect them? And how do I change all the contacts on my computer into mine? We share the My Devices section… both of ours are the same under both IDs …. And my iPhone isn’t under either of them. So confused