So... "como vai?" and "como está?" have the same meaning: "How are you?"
In Portuguese it's usual to answer these questions with "beleza", or "tudo bem, e você?"... it's kinda like in English.
"E aí" is the Brazilian version of "What's up?" haha, then you can answer like Denis said "Estou bem", "Mais ou menos", (...), "Não tão bem"...