You can say:
Ne haber? (pronounced n'abér) = Lit. What's new? Meaning: What's up? What's new?
Ne yapıyorsun (also pronounced n'apíyon)/ ne yapıyorsunuz? = Lit. What are you doing? Meaning: What's up? What have you been up to (these days)? How have you been?
Ne var, ne yok? = Lit: What is there, what is not there? Meaning: What's up? What's new?