Year 12s: what will be the first way you research your future options? >>

How can I politely decline a shift, or better, quit my part-time job I just started?

Just over a month ago, I went to do a trial shift at a local restaurant, then last week I did my first proper shift there. I really hate it, like I constantly dread having to go back. At first I wanted some money, but now I'm honestly not bothered about having none, just as long as I don't have to go back to the restaurant. This afternoon I got an email from one of the managers, asking me if I was able to do a 'kitchen porter shift' tomorrow afternoon. So far I've worked as a waiter, but from what I've seen and heard of the kitchen, I can't imagine this will be any better. Is there a way I can politely say no without sounding too rude or annoying the manager? It feels like it's short notice enough for me to fake some excuse, but I'm not sure I'm obliged to go. Alternatively, if anyone could suggest a way for me to say that I've realised the job is not for me and would like to quit, or something like that, I would be so grateful, as this is really becoming a lot of stress which I don't need at the moment. Ideally I probably need to reply within the next couple of hours, so I would really appreciate it if anyone could give any suggestions soon. Thanks guys!