For singles, I suggest you stick to a 3, 4 or 5 day cruise. The Bahamas/Cozumel cruises on Carnival will have the highest percentage of "under 30's". Most of the organized single cruises will have people just a bit older than you.
As a single man in his 20's, you will not be alone very long on the Fascination, Fantasy, Imagination, Sensation, Celebration (all Carnival ships).
I would take an early/mid fall cruise because there will be less children and less teen girls pretending to be over the age of consent (major problem). Go after school starts and you won't take a chance on running into that type of trouble. On that note, check out any young lady's sail and sign card. If she is under 21, her sail and sign will show it.
You will have an awesome time. After you get your sea legs, you can try an organized group cruise.
As far as going with a buddy. . . do what works best for you.
I B CRUZIN'
(solo cruzin' by design)
CCL Victory, CCL Imagination, CCL Victory
Past Cruzes: RCCL Navigator, RCCL Majesty, CCL Destiny,
Glory (2007),Liberty (2007), Fantasy (2007), Destiny (2006), PofAm (2005), Inspiration (2005), Fascination (2004), Victory (2004), Fascination (2004), RCCL VOS(2003), Fantasy (1999)