When should I ask her out?

I am generally nice guy (far from perfect, but courteous, etc) and freshman in high school. Girls is there any timing to asking someone out because when I'm not nice or in a good mood I'm really cynical and the girl I would ask doesn't always like that so if I was in a stretch of a couple of days acting like that would I have less of a chance if asked her then rather then asking her when I'm behaving very nicely?