I'm very well and you? (we would say - Je vais très bien)
It means 'very well thank you, and you ?'.This is the standard reply when a friend asks 'Comment ca va ?' (how goes it?)
"bien, et toi ?" is the answer people make when they are asked "how do you do?" It means "fine, and you?"
It means- I'm fine thank you, and you?
tu écris trés bien - you write very well