This information is no doubt recorded elsewhere on the web, but I might as well write about my own experience.
I don't live in the US and I don't have a US-based credit card. My PayPal account is linked to my non-US credit card, and therefore is not recognized by iTunes as a valid US-based payment method. For this reason, iTunes refuses to let me download anything from the iTunes US Store -- not even the free apps.
The easiest way to create a US iTunes Store account is to buy an iTunes Gift Card and use that as the primary payment mechanism for your iTunes account. Since I was in the US recently, I picked up a handful of gift cards at the nearest brick-and-mortar retailer and used that.
Note: This process assumes you already have an existing iTunes account that's valid for a different country.
- Launch iTunes and login using your existing Apple ID.
- Click "iTunes Store" in the left sidebar
- Look for your Apple Id in the upper right corner. Click that.
- In the pop-up that appears, enter your existing password and click the "View Account" button
- In the new screen that appears, look for the "Change Country or Region" button. Click that and choose the USA
- To successfully create an iTunes account for the US Store, you need to enter a valid payment method. The initial set of options allow you to add a credit card or a Paypal account. Select "None"
- The second section on the screen lets you redeem an iTunes Gift Card. Click the "redeem" link and enter the code on the back of your gift card.
- You'll be required to enter a valid US-based address, which will be the basis for calculating Sales Tax for your iTunes Store purchases. I used the address of family who live in the US. If you're really interested in keeping your sales tax low, you may want to review this sortable table of tax rates for various US States.
- Scroll to the bottom of the screen and click Done. You now have a new account for the US iTunes Store, using the same email address and password as your non-US iTunes account.
- Test that you've done this correctly by downloading an app that's only available in the iTunes US Store. My test app was the free Scrabble App from Hasbro/EA.
- Sync your device (an iPod in my case) with your iTunes account to confirm that the newly downloaded app is successfully transferred to your device. Note: All your previously downloaded apps will still remain in iTunes and on your device.
- Now attempt to download or update an app that's already existing on your device by going to the App Store app on the device itself. You'll be prompted to switch from your current country to the US iTunes Store. Just say yes, enter your Apple ID password, and you'll be switched over.
And that's it. You now have a US Store iTunes account with a small usable balance. If you need to replenish it, you'll just need to buy new iTunes Gift Cards.