We have done 20 Ocean cruises and 4 River cruises in the past 9 years.
Cruises are great for visiting lots of ports and you are in your floating hotel with entertainment, etc.
Especially good for visiting ports that are a bit hard to get to otherwise, like the Greek Islands or Dubrovnik.
In picking a cruise and cruise line, competition is you friend. Compare several lines with their prices. Some of the cheaper cruise lines like Costa and MSC are Italian lines and don't rate so high in customer satisfaction.
Lines like Viking (they have river and ocean cruises) for ocean cruises are great, but expensive.
Lines like Carnival appeal more to younger people and people that drink a lot.
The lines that we have used are Celebrity, Royal Caribbean and Norwegian Cruise Line (NCL). They are all good, but Celebrity excels in dining while Royal and NCL excellent in entertainment.
Princess and Holland America (HAL) are good lines. We avoided HAL because they had a very liberal smoking policy (smoking on balconies) but they changed that policy to be more in line with the others a couple of years ago.
Do some research, compare ports for itinerary and pricing. Some lines tend to bundle promos like drink packages (alcohol) with the price, but you can usually avoid that if you don't want a promo.
Price wise, if you book early, soon after the cruise is announced, you will get a better price, except that sometimes you can look for last minute bargains. If you are flying from North America to Europe, you probably want to book early.
The website cruise critic.com has cruise reviews and boards with posts from people that cruise. It is a great resource.