single room selection

i am a current first year student who just got a single room selection time for next year. i am just wondering how it works when i go to select my room so i am prepared. is it like a list of specific rooms or just the halls and then they assign your room from there? what are the odds that it opens when my time comes and there is nothing left? i'm sorry for the perhaps naive questions, i just can't find very much online about this and would really love a single.