Anything will do, really the only requirement is that you don't have to be there all the time, so this is more geared towards the kind of job where people can be replaced on short notice without interrupting the workflow.
Think retail, that sort of thing. All you need is for it to cover the basic expenses and to leave your head free so you can think about stuff while doing the work. Then when you get home you're not too mentally tired to do some 'real' work for a bit, and on your 'off' days you can implement the stuff that you've been thinking about / working on on a slightly larger scale.
When I was 17 or so I worked in the mail room of a bank while at the same time working hard on becoming a professional computer programmer. If the work had been more demanding mentally that would have been a lot harder.