Talking to girls was the hardest thing i went through in my late teens to early 20's.
First of all you have to try (it helps with a few drinks or mates to back you up and egg you on).
The first rejection is the hardest, once out of the way you realise it sucks but is not the end of the world (for the record i never copped a slap but some girls are straight up bitches).
One theory a mate of mine had is that for every rejection you are closer to a yes.
Remember all you are doing is talking and the girl is as nervous as you so smile, stay confident say hi and find a common topic.
Once this is achieved then you can sus out how much they are interested in you (you will learn this by experience).
Finally and most importantly if you get this far do not forget one thing.....
get a phone number/email address or else the whole thing is a waste (the reason i say this is too many of my mates get going and just forget to finish) .... in basketball terms its like going up for a dunk and just forgetting to dunk the ball
Now do you think you will see this girl again?