Can I use my old Mac's screen with a Mac mini to migrate data?

The Mac mini is a computer. It has a processor, RAM, an internal SSD, Wi-Fi, Bluetooth, and ports to which you can attach accessories.

It doesn't include the keyboard, mouse, or monitor. You need to provide a keyboard, a mouse, and at least one real standalone display.

If by "the screen from my old Mac", you mean a standalone display, chances are that there is a way to connect it to a new Mac mini. You can't use an iMac's built-in screen as a first-class hardware display for a new Mac mini. Even if your iMac is old enough to support Target Display Mode, current Macs do not support using any iMac as a Target Display.

jsn320 wrote:

i am trying to understand what the mac mini is?

The Mac mini is a computer.

see > Mac mini - Apple

can i use the screen form my old mac and work with the mac mini ?

No you need to connect and use an external display with the Mac mini.

see > Connect a display to Mac mini - Apple Support

how do i migrate etc

The fastest and easiest way to migrate, is using your Time Machine back up.

see > Transfer to a new Mac with Migration Assistant - Apple Support

Your iMac cannot be used as a normal monitor with a new Mac Mini. I would recommend that you make a Time Machine backup of your old iMac and use that to migrate all your data when you setup your new Mini.