I've worked in a YOC, and I can only echo what wizard says. 'Porridge' is an old, stereotyped programme now, but Fletcher's line, "bide your time" still holds true. I've never worked in an adult prison, but I wouldn't think it's very different except in terms of age of inmates.
Expect the officers to be very strict on you to start with - after all, you'll be an unknown quantity to them. They can't afford to give you any leeway. Don't cheek them. Do as they say and question later. Surprisingly, they're quite human in the main. We used to have an older officer whom most people disliked - the epitome of Mr Mackay - but lads for whom he was key officer (your assigned 'personal' officer) couldn't speak highly enough of him.
Expect to be locked in your 'pad' (your cell, which you'll probably share with at least one other inmate) for most of the day, unless you take work or go to education classes, and expect spot searches at any time. Behave, and you get credits, which you can save for priviledges like a radio in your pad. Misbehave, and those priviledges are taken away.
You should be able to buy toiletries, cigarettes and sweets, but you won't get much money, so budget carefully. I've known lads to buy their smoke allowance even though they don't smoke, just to have the ciggies to trade, but I don't know how 'legal' that is.
Also, you're unlikely to be allowed free access to newspapers and internet facilities, so be prepared for that.
Pixie, I don't think this is a wind-up. I've worked with lads of 20 and 21 in YOCs. Sometimes there isn't room to transfer them to adult prisons, and sometimes it's felt that it wouldn't be a wise move. Mark is still very young in prison terms, and as a first offender, committal to a YOC is a very likely possibility.