Do a search on any search engine (yahoo, etc.) for cruise jobs or cruise careers and you will find info on there employment agencies, etc. Also you can look at individual cruise line sites most will have career information.
I worked for Clipper Cruise Lines for 6 months. They're a very small line with small ships, so they can go to more remote places than the big ships - which makes it more interesting for the crew as well. They hire Americans 21 years and over. I tried other lines as well, but with little experience, only Clipper responded.