Its hard to explain but the way she carries her self is the most important. There is a certain confidence that she will exude that makes me actually attracted to her rather than just wanting to hit it.
Now a chick who I just want to have sex with would be, for instance, the girl who is wearing short shorts a tank top and high pumps to school or a really short skirt and tons of makeup for no reason at all. Its easy to tell when a girl is trying to hard. Just be comfortable with what you have and we will love you for it.
The face first - more specifically her eyes. Some women just have amazing eyes that I just love looking into when talking with them. Not in a creepy way though, obviously :P
After that I usually look at her ass - can't help it, a curvy woman is really attractive. I don't like women who wear super super tight jeans though. They make the womans gut stand out and how can women walk with those, or even change in the morning? I always have the vision of that Seinfeld episode with Kramer and his ridiculously tight pants when I see a woman wearing something way too small.
As far as weight goes, not too skinny but not too overweight. Can be a bit below or above average - id actually prefer average or a bit above average.
One of those cute shy faces. She gives off a nice smile when she thinks something funny will have me falling all over a girl. She doesn't need to have a model type body. I like girls that are average maybe a LITTLE past average in the weight and curvy side. Boobs are probably the last things I look at, but if she has a some booty =) now that's atrractive.