"No way!" is a casual phrase that means "I can't believe it!" Use it when you're surprised and excited about something:
A: Have you been playing the new Call of Dutygame?
B: I already beat it.
A: No way!
You can imagine that "No way!" is a shortened version of "There's no way that could be true."
Another meaning of "No way" is "No" or "Definitely no."
A: Do you want to go skydiving with me?
B: No way!