sexy is pretty much an attitude, as Meghuney demonstrated. Some of the sexiest people out there aren't necessarily that pretty. I never thought Kim Cattrall was especially good looking, (she's OK) but she always portrays herself, on Sex and the City or in real life, with supreme confidence and extreme self-love, and people are attracted to that. It has very little to do with size, age, colouring, weight, race etc. even though the media would have us think otherwise.
But in order to be sexy, you need to know yourself, make peace with yourself, and be comfortable with yourself - so what if you don't have a 6-pack stomach or big boobs? (you as in generic you....)