In one of the lessons, I don't remember which but it was said that the subject of the sentence is implied unless otherwise stated. In other words you would not say Anata wa o-genki desu ka you would just say O-genki desu ka?
That being said, I just went over beginner lesson 4 and it said that to say "I am busy," you would say Watashi wa isogashii desu. I am wondering if it would be correct to just say isogashii desu. I am also wondering about this when asking a question. Say I walk into a room and there is only my friend there, Would I simply say "Isogashii desu ka?" or would it be "Anata wa isogashii desu ka?"