My first job was actually with my Dad. My parents ran "Season's Greetings", where Dad was hired as a professional Santa. I started working as an elf, then as Christmas Carol, because babies and small children were more apt to let a small female hold them for pics than the hairy scary dude in the big red suit. I did that every November-Christmas Day for 6 years.
The first non-seasonal job I got was when I was 16. I got hired as a hostess for a steakhouse restaurant. It wasn't as much fun as working Christmas parties in costume.