I just got a new atiel and it's untamed?

I just got a new atiel today. My family member knew a man that wanted to sell it to us for a cheap price. When I went to the place, the bird was in a cage that was too small for it. There was no food and water in the cage, but the man said thats because he just got the bird in today. The cage was disgusting and dirty. The birds feathers arent in great condition from being rubbed up against the bars of the cage, and near the nose it looks like the bird got scratched and theres a little blood. So, I immediately bought it a new, much larger cage and filled it with toys, food, and water. The bird hasnt been handled before and needs to be trained. The man says once the bird gets acquainted with the home that I should leave the cage door open so it comes out, but it bites and I don't want it flying wildly around (it's wings arent clipped yet). Also, since I put the bird in its new cage it seems very frightened and it hasnt even eaten or drank anything yet. It just stays in one spot. I feel so bad, I wish there was something I could do. How can I train it to come on my hand and feel comfortable with me? By the way, its about 3 months old.