Well when a woman has curves, evolutionarily it has been a sign of high fecundity, which is something very desirable in a mate. It also shows that the woman has a certain amount of fat stores that will help them survive throught the winter and are not malnourished. Sure some of those things aren't so important now, but men's subconscious minds don't know that.
You can act in a feminine manner and that would make you feminine. Physically a woman with curves may be considered more sexually desirable, but some men find waifs more attractive, so don't worry about it. If you are under 21 then you aren't even fully grown yet, and still will develop. If you're older, just because you don't fit the physical ideal, does not mean you are unfeminine or unattractive. Don't worry about it.
Edit: yeah...Marilyn Monroe was soooo "fat." I never saw a Playboy spread titled, "The Women of Auschwitz." The only men that like the way anorexic "supermodels" look are the gay fashion designers that pushed that look on the public.