Firstly, you need to buy an iTunes gift card for the US store, I recommend:
Overall, PC Game Supply looks more professional, but I’ve often had problems with the order or credit card, so now I use Jerry Cards exclusively.
Both of these sites will contact you when you try to put your first order through. PC Game Supply will phone you, but Jerry Cards will accept a Facebook, twitter, Google+ or Skype Message.
Next, launch iTunes, go to iTunes Store, scroll to the very bottom and you should see a flag, click on it and change it to the US store.
When done, go back to iTunes Store, and in the top right hand corner, click on redeem and type in the code from the gift card that you bought.
They might ask you for an US address, if so, you can get one here:
Don’t give iTunes your credit card number, if they insist on one, you’ll need to create a new account, and start again.
There is another method, but it didn’t work for me. You can theoretically call up your credit card company and ask them to add a second address to your account, giving them an US address, the fake address above should work. All credit card companies will do this. Then when iTunes tries to match your credit card with your US address it will work, and you should be good to go. It’s worth a try doing this (if you have time to wait on hold with your credit card company), but it didn’t work for me.