Firstly, you don't need a credit card and this is one of the easiest ways to get a US itunes account. You can use this method to buy anything from the US iTunes store even if you are overseas and nowhere near the US. You can even buy things like Apple's iWork suite or even tv shows like Lost or The Office.
1) Log out of your iTunes account by opening iTunes and logging out.
2) On the iTunes Store at the bottom of the page, you will see a "change country" button, or you can also click on the country flag.
3) Select the US iTunes store by selecting United States with the Red, White and Blue flag.
4) While in the US store, visit www.itunescarddelivery.com and buy your first US itunes gift card! While that codes being delivered, download any free app, the first free app you see.
5) Select the app and try to download, when Apple asks you to sign in, Create a new account instead.
6) Continue to create your United States itunes account
7) Accept the terms and conditions
8) Fill in with your email (that hasn't been used in the app store before) password and security questions.
9) This window will now ask for you to use a credit card to register your account, but you don't have to!!, simply enter in your US itunes gift card code and you can immediately be accessing the USA itunes store!
10) SWEETNESS!!! You now have a 100% working United States, US, iTunes store account. This account can be used to download anything you want now!
You can now download any free or paid tv shows, apps, movies, or music from the US itunes store.