You have to work at something but it doesn't have to be at the Ministry. There are people who make things and fix things and sell things. I've thought about trying to go into business making things like jewelry and fancy scarves and so on. That wouldn't be a good one to write about, though. Working at the Ministry would be a much better one to write about.

I suppose you don't have to know exactly, you could just talk about how you want to Serve Our Lord as directly as possible and devote yourself to a life of public service like he has (Director Selwyn I mean). People get moved around departments all the time, I think, when they go to work at the Ministry. The Slytherins who do, at least.