The narrator says finding the ring is a "turning point in his (Bilbo's) career." Why?Gollum accuses Bilbo being a thief. Do you think Bilbo is? Why or why not?
Up to this point, Bilbo hasn't felt that he had any advantages. He is out of his depth, out of his own world, and dealing with things that he has never dreamed of. He is afraid and lacking in self-esteem. But having the ring gives him a sense of power, and certainly gives him some benefits. After getting the ring, he becomes more of a leader, and is able to escape the goblins and - later - save his friends in the forest.
As for your second question, you have to decide. Bilbo knows that the ring belongs to Gollum. If he is justified in taking it because he has to protect himself, then you could say he isn't a thief. However, you could also say that - no matter the reason - Bilbo took something that doesn't belong to him so he is a thief.