I can't sync my iphone 5 to itunes?

My father recently bought te new iphone 5, then he gave his iphone 4 to me. Somehow everytime I download an app from itunes store it automatically download on my father's phone as well. Is it because we're using the same apple id or probably because we sync in the same computer? If so how can I download an app without auto download on my father's iphone? My father also can't sync app from itunes in our computer

On your iPhones go into Setting > iTunes & App Stores > Under "Automatic Downloads" make sure its switched to no.