"How are you?"
What about difference between: Io sono bene (does this even exist)? Io sto bene
Yes, it's easy to think "come sei tu" would be ok but sadly it's not. The verb stare is used when discussing personal states of being, not essere. "Come stai" = (familiar) how are you. "Sto bene" = I am well, "Come stanno" = how are they etc.
there are many phrases for asking how are you, like people have written, come va, come stai, come stanno, tutto a posto etc. but i am a bit curious about come stai, why do you say it like that, is it an abbreviation of something ?
"Come stai?" is a direct translation of How are you? Stai is the tu (you, singular, familiar) form of stare (to be as a condition). It is not an abbreviation. The other verb to be "essere" is used for description. "Come va?" is like "How's it going?" "Come stanno?" would be "How are they?" "Tutto a posto." is like "Everything's fine."