how to remove junk file from Mac

Are you talking about emptying the junk folder in the Mail program?

From the Mail Menu Bar at the top of the screen, select Mailbox, pull down to Erase Junk Mail, a popup window will ask you to confim you want to delete junk mail, click the Erase button.

If that's not what you need, perhaps if you'll provide us with more specifics like iMac Model Identifier from the About This Mac, More Info window, the version OS you're using, and what kind of file is it you want to delete, and where it's located on your iMac, we might be able to provide more assistance.

Good luck.