Solo cabins are often much more than a good deal on a regular cabin. Every cruiseline asks how many occupants per cabin, just default to one. I don't know about the single supplement? I just double the amount listed for one person. A solo doesn't pay double the port fees and taxes. The cruiseline itself will do the math and tell you what you'll pay at the end of the process. Go all the way to the final amount for that cabin and see if that's what you want to pay for your trip. If I see a cruise advertised for $545 a person, Oceanview cabin, I know I'll pay about $1100.00 plus taxes and port fees for one. I've been on two cruises with two different cruise lines and that's always how it has worked.
PS that's how hotel rooms work too. You pay for the room regardless of whether there is one or two occupants. Good luck! A cruise is a great way to travel if you are traveling solo. It's very safe and can be amazing.