How does college work?

I accidentally put the wrong ssn on my GED test and as a result, i'm getting my scores two weeks from now, when everyone else got theres already. I want to do three high school credits to get my diploma in the meantime, but a college i wanted to apply has its deadline today! I dont understand why I cant apply for school anytime, like in high school. I dont understand all the college lingo and why college is so different than high school. I think i'd be an undergraduate because im 19 and have never been to college. Its a real bummer i've missed the regestration deadline for a few choice colleges because i'll be 20 by the time fall term begins at FIU. Explain?