How to get a job whilst young??

Helllooo, I'm 14 and I wanna get a job so I can eat and all that fun stuff! My mum is my only parent and she can't work so we live of welfare and all that, I want to get a job at my local shop but idfk how to go about it. They did recently sack someone so there is an open space and I've helped out around there when I was younger. So how would I get a job... and how do I warm my mum up to the idea of it? Anyway thanks in advance if you can help <3