Tanya, go to the Singles Section of Cruisemates and read my article on cabin mates.
BTW, if you go on an organized singles cruise, like our 4-night 4th of July Cruisemates cruise out of Miami you would be paired with someone of same sex/age and smoking preference. Check out the details on the Singles Cruise Calendar.
I've done the single share one time on Carnival. It actually turned out to be a good experience. She was with her family on the cruise and I really didn't see much of her. But she was a very nice person and also a considerate roommate. Though I didn't run into any problems, it did make me realize that I prefer a cabin to myself. It can be kind of awkard sharing such a small space with a stranger