What do i need to tell her?

On of my friends who went to prom with me is who I am after. We have known each other about 7 months now and are just friends, but slowly becoming better friends with her. In fact, she invited me to go to a concert with her in a couple months. Yes I am paying i think, but still she invited me. Only thing is we dont talk but like once a week or even hang out but once every ten days or so., because when I texted her saying whats up every other day I heard she was getting annoyed, and I cant tell if she likes hanging out wih me. But I want to be more than friends with her, and dont know what or when to say it.