how to access my iTunes stores account?

What exactly do you want to access/do with the account ?

You can log into your account on your phone via Settings > iTunes & App Store, if you want to view/change details on it (e.g. payment details, primary email address) then (and if already logged in) tap on your id in Settings > iTunes & App Store and then tap 'view apple id' on the popup. To buy an item from the store store tap on the item's price or free button in the iTunes or App Store apps

On your computer's iTunes you can log in via the Store > Sign In menu option (if already logged into an account then that will show as Store > Sign Out), and to then view/change the account's details use Store > View Account.

How do I change stores back to the US? I watched a video that tools me to the U.K. Store and I want to make sure I'm back in the US store.

If you have US as the country on your account then try logging out and back into the account on the device/computer that you are using