What is the Chinese equivalent of “to pick up a girl” or “to hit on girls”?
Off the top of my head, I can think of 勾搭, 泡妞, 把妹 and 溝女


  勾搭 
  
  http://www.cantonese.sheik.co.uk/dictionary/words/52068/
  
  [1] pick up a lover


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  溝女 (Cantonese slang)
  
  http://www.cantonese.sheik.co.uk/dictionary/words/20482/
  
  pick up chicks; go out looking for women to chat up; dawdle away one's time with a girl; [slang] cruise (look for sexual partner)
  
  [variants] 媾女, 勼女, 追女仔
  
  溝 is most probably a loanword from '(to) court'. 


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  泡妞 (Mandarin slang)
  
  to pick up chicks; to go out looking for women to chat up; to chase skirts; to dawdle away one's time with girls |
  
  http://www.cantonese.sheik.co.uk/dictionary/words/40924/


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  把妹 (Internet slang)
  
  http://www.cantonese.sheik.co.uk/dictionary/words/58867/
  
  to chase girls; to pick up girls 


 I'm going to go and pick up some girls!


我要去泡妞 (Mandarin slang)

我要去溝女 (Cantonese slang)

我要去把妹 (web slang)

我要去勾搭女人 (literal) "女人" can be replaced by 女孩 or 女生 in Mandarin and 女仔 in Cantonese