Clearing out my old Mac for charity

I want to give away my old Mac to a charity, but I don't want there to be any of my files on it. But I am worried that if I delete them they will be deleted from iCloud too. I do have a time machine thingy that is hard wired to my new Mac. You can tell by this message that I am not computer savvy so if there's any help or advice on offer please don't use jargon - I am quite old.