I would suggest going in to Yahoo Travel or Expedia, any of the on-line travel sites. Plug in the dates you're interested in going. Then you can compare cruise lines and pricing, etc. Personally I wouldn't use those to book, I'd go with a cruise specialist travel agent. But you'll get a good idea with these sites on cabin pricing and availabilities.
You will have too many choices so you'll quickly need to decide on what you want to do or can afford. If you want to do a land tour to Denali, etc you should choose the one-way cruise. I would suggest if you won't be doing a land tour then the round trip to/from Vancouver or Seattle is the way to go. It'll be cheaper that way and more convenient.
IMHO for a first Alaskan cruise find one that visits Glacier Bay. This would also help you focus on a cruise itinerary and cruise line to book with. Celebrity for example doesn't go to Glacier bay.
Vancouver trips go much more into the inside passage and it's really quite spectacular.
Seattle trips go outside of Vancouver island and less inside passage. Still quite beautiful and Seattle's a more convenient port if you're traveling from the U.S.. I prefer Vancouver however.
You are not too late as mentioned above. Start doing the research to narrow down your needs, wants, and budget. The research is actually fun IMO.
You've made a good decision. Alaska is awsome.
