So you want to ballroom dance? You're about to enter a world of class, tradition, and loads of fun. The good news is it's easier than you can imagine. The bad news is you'll still need to work. Ballroom dance is a somewhat artificial category including many styles of traditional partner dances from several parts of the world and different eras. The thing in common is the expectation that the dance is polite and the dancer's dance-floor relationship somewhat formal (Ballroom dancing is never "Dirty Dancing"--even the more sensual styles such as the Tango are stylized. The sexuality of the dance exists in innuendo, not in hot and sweaty contact between the dancers).
Ballroom involves multiple styles
While you don't need to learn all of them you will need to learn a few core styles at first. Complicated dances such as the Bolero and paso doble can be picked up later, but if you want to go out dancing and not be stuck in a chair the whole night you'll need to know the rumba, cha-cha, tango, waltz, and fox trot.
Decide what you want to dance for
Do you want to do social dancing or do you want to compete? Do you want to go out each weekend or do you just need to look good at a wedding? A specific type of event might require that you learn only a few styles but you'll want to learn them in depth. You'll be better prepared for a night of casual social dancing if you learn basic steps for a number of dance styles(you will pick up more moves as you go out dancing, practice and the basics become familiar).