Pick a girl , act normal and relaxed when she is near , talk with her , say few jokes , if you feel something in her look / eyes / smile you can ask her to go to watch movie / walk / or just go for a drink or something... If she accepts you should get into work , ask her what does she like and what is she doing these days , if you find something in common its great , etc... be creative , buy her something cheap but self-made or something , be gentleman if you are in restourant or something (but you are 15yo so don't go to restourant :D )
What you should do ? Ask your friends for advice , if they are more experienced than you.
What you shouldn't do ? Ask her friends what does she thinks about you before you talk with her more than few times , because they will tell her and you will look like someone who is scared to talk with her (amateur) or something :)
Good luck little one ;d
Get out, meet girls, talk to girls, flirt with girls, become friends/more with girls!
Really upto you :) Just get out there and start talking to girls, and get to know them. If you find that hard I recommend looking up "Simplepickup" on youtube as they provide educational and fun tips etc on this matter.