I've been in one before, but only for 6 days. I was on probation when I was 11 years old - some crap my mother pulled - and I didn't make good enough grades for my probation officer. So, yeah... I was sent to one for 6 days, even though it was supposed to be 15; it was 6 days away from Christmas, and they felt bad I suppose. Anyway, enough of that.
It's mind-numbingly boring. You do have to wear a uniform. The cell I was in held two people, the bed was a 1" thick mattress laying on a huge cement block. There was a window, probably thicker than my mattress. There was one toilet and one sink. **** was written all over the walls. They made us shower in pairs of two, separated by one wall, but ultimately, we were open for anyone to see. They watch TV and play dominoes, and may occasionally have a preacher come in. They will probably make you do school work on a computer as well. I believe there was a basketball court, but it was too cold to play.
And this is not to scare you, but I had to listen to someone being raped; it was brief, and the guards stopped it, but it was still pretty traumatic. If you're going, tell them you've tried to kill yourself, because then you may get a room alone.