What am I supposed to write in a college recommendation letter?

A kid I supervise at work asked me to write him a recommendation letter for college. I’ve known him about a year or so, but not on a personal level really. We talk at work and joke around, but other than that, I don’t know much about him. I have put off writing the letter until it’s nearly too late. I asked him what it was supposed to say and he told me just to write about him-not helpful information. I told him to write it and then I would copy it and sign it, but I don’t think he thought I was serious. Does anyone have any suggestions about what I am supposed to put? I’ve never written one before and I’m really lost!