I’m sorry but can’t use my new iMac as can’t connect mouse ! Please help x

Did you try the steps in this article?

If you can't connect your Magic Keyboard, Magic Mouse, or Magic Trackpad to your Mac

What mouse do you have?

If it is a new Magic Mouse that came with the iMac, connect it to the iMac using the supplied charging cable.

If it is an older Apple wireless or aftermarket wireless mouse, then you need to dig up an old USB mouse in order to pair the wireless mouse.

Make sure the mouse is turned on with the button on the underside.

You’re welcome.