We've been on many cruises.  The quality and experience will depend as much on the cruise line and the particular ship as it does on the details of your booking.  In general, I would say get a room with a view (either a window or a balcony as Hard_Eight mentioned).  Food varies greatly between cruise lines.  Most lines offer "free style" cruising now which is basically much less emphasis on formal seatings for dining.  Another fairly recent trend is "pay as you go" premium dining, which offers an upscale experience (I don't think it's that upscale actually) over the free meals that are included with your ticket.  We have cruised on Carnival, NCL, Holland America, and Royal Caribbean.  They are all different and Carnival and NCL in particular have changed significantly over the years.  As a first time cruiser, I'd recommend you stick with a shorter cruise, maybe one of the standard Bahama cruises they offer from Florida.  In fact, the Carnival ship that sails out of Jacksonville does 4 and 5 day cruises.  Carnival tends to focus on "value" cruises but the shorter trips on that ship are really relaxing and although it's an older, smaller ship it's really pleasant in an understated way.  I would recommend it as a very mellow, laid-back kinda cruise unless you're looking for a major party boat.

 

A couple other notes, don't forget your passport.  Book excursions through the cruise line (these aren't necessarily the best excursions, but they ARE backed up by the cruise line and they guarantee they'll get you back to the boat before it departs).  Also, you're highly unlikely to get sea sick.  The big cruise ships hardly even twitch but you might want to take some dramamine just in case!  

 

Edited to add: I just looked up..the ship out of Jacksonville is the Fascination, that's the one we've been on but they are replacing it soon with the Elation.  Same class of ship but a bit newer so YMMV.

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