courtney, at your age and looking for singles of similar age and intersests I'd say you'd be best served to look at Carnival and Royal Caribbean. Look for a week or less and western caribbean or Bahamas seem to draw a little younger crowd.
NCL has a few newer ships that are now drawing a younger crowd as well. The pearl, their newest ship seems to have all the goodies that draw younger people (water slide, rock wall, etc.) including a bowling alley, the first on a cruise ship.
Whatever you decide, get back to us and let us know what you chose and how you enjoyed it.