Carnival is usually very good at placing solo travelers with other singles for dining, families with families, etc. Your first line of action should be to have your travel agent request specific seating before the cruise. If, however, you find you are not happy with your seating once on board - speak to the maitre'd and explain your situation. If at all possible, your request will be honored.
Check out Nancy's page at cruisereviews.com. There is an article "Solo at Sea: Seeking Others in the Same Boat" that addresses how and where to meet other singles onboard. (I really have used the suggestions -and they work!)
Enjoy your trip - Nancy