ok first of all you need to set your sights on someone . texting is probably the easiest way to get in there or internet messaging but some girls prefer calling , dont worry they will ask can they ring you and its grand like , just chat .
if you wanna "get" in there you need to test the water and see if they like you too . you can flirt a little at first and if she replies positively then thats usually a good sign
personally the last time i got a girl i was texting her for like 4/5 days and one phone-call . we had sex on the first date but it doesnt always go as smoothly .
to ask her out on a date i suggest you first just generally find out her interests and stuff and then come up with something from there , e.g if you both enjoy going to the cinema then just casually say " hey , did you hear _ is out now ? you wanna/going to see it ? " ... this is pretty safe because if she says yes then maybe say you wanna go too or even suggest how about going with you . if she says no then its ok too because you didnt actually ask her out with the question , you were just conversating .
there is no limit to how well or long you need to know them , girls are different . some girls are very forward and can make it easy because they might suggest the date for you . some girls may flirt a lot but beware they may be "wanna-be's" . these girls are the worst because they act like they're into it but they are usually quite innocent really and not worth the effort if you arent looking for anything serious . they play games and try to have the upper ground , basically like they are in control . also you cant control the situation or her , it has to work . if it doesnt dont push it just move on
you are officially dating when you have met more than once , are hanging out a lot and texting / talking regularily and are physical in some way . plus she will tell people you are her boyfriend and probably say " would you ever cheat on me ? " if this is said then she considers herself with you . if you want a girlfriend remember she has to be your friend too not just someone you fancy .
this is just a few things i know from my own experiences , its a game really . sometimes you win , sometimes you lose .