Changing from Hong Kong to USA

I bought my Macbook Pro and Iphone 4S when I was deployed to Afghanistan. I used them for the whole time I was over there. I am back in America now, and in my app store every app I go to purchase is in Hong Kong currency. How can I change it to the U.S Dollar? Also it wont allow me to download certain apps because it thinks I am in Hong Kong. How can I change this so it will work?