Lots of wonderful advice has already been shared above - this is what works for me:
I carry a book (or e-reader) and/or my needlework everywhere when cruising solo - everyone interested in either will stop by and chat, whether it's in the theatre waiting for a show, the buffet, on deck, or anywhere I find a nice spot to settle in. (They want to know what you're reading, what you're making, etc.)
I'm quite shy so having other people initiate the conversation really helps me out - I have met so many nice people!
Best wishes to you, enjoy your cruise!