How long should i wait to ask her out after they break up?

ok shes 15 and im 16 i hang out with her more than her bf i dont think theyll last .1..so wen they break up how long shoult i wait to ask her out and 2.. wen i do should i just text her if she wants to get something to eat or just to go out and hang out /or should i tell her how i have always felt about her and that i like her PS shes a friendly and nice girl and ive seen her old boyfriends and some r ugly and some r weird so i think that would help not that im ugly