The single most important step in selecting a cruise is do your homework. Take the time to understand the options as far as ports, ships, and time of year. No two cruises are exactly the same and depending on what is import to you one persons recomendation could be your nightmare.
Same cruise "we loved this ship with 900 other kids onboard the family had a blast" , "this was the worst cruise out of 30 with 900 kids running wild all day and night"
Get a good book like this one, go online and read as many reviews as posible, use message boards to get clarafication, talk with friends that have been on cruises, and if you are still unsure find a good Travel Agent that has been on the ships and to the ports you are considering.
Once you have narrowed the list it becomes a simple exercise based on price, intinerary, date, and ship.
For us, itinerary and price are our tops concerns, followed by what the ship has to offer. Every cruise line has a nitch, read all you can, talk to others, keep an open mind when reading reviews and if this is your first cruise I do recommend a travel agent.
There is fun in doing your own research, but a good agent will know what questions to ask, will be able to read between the lines and help you decide what fits into your needs and desires.