The most common and informal way to say "how are you?" in Italian is "Come stai?" This phrase is used with friends, family, and people you know well. For a more formal situation, such as when speaking to someone you don't know well or an authority figure, the phrase "Come sta?" is used instead. Other common informal alternatives include:

• "Come va?" meaning "How's it going?"
• "Tutto bene?" meaning "All good?"
• "Come te la passi?" meaning "How are you doing?" with a more personal touch

These phrases vary based on the level of familiarity and context but "Come stai?" is the standard informal way to ask "how are you?" in Italian.