Verification Failed: Your Apple ID or password is incorrect / This Apple ID has not yet been used in the iTunes Store

When I try to sign into the app store, it asks for my Apple ID and Password, then asks for my password again, and then gives the message "Verification Failed Your Apple ID or password is incorrect." I've reset my password; but it will not accept the new password, either. I've tried restarting my device, and I've tried disconnecting and reconnecting to the internet.

When I try to download an app, I receive the message "This Apple ID has not yet been used in the iTunes Store. Tap Review to sign in, then review your account information." Selecting "Review" brings a new pop up that says "Sign In" with the two options below being "Use Existing Apple ID" or "Cancel." Selecting "Use Existing Apple ID" brings the pop up "Sign in Required. Please Enter your Apple ID and password to create an iTunes Store account from your existing Apple ID," followed by text boxes to enter your Apple ID and password in, and two buttons below that are "Cancel" or "Sign in." When I enter my Apple ID and password and select "Sign in," the original "This Apple ID has not yet been used in the iTunes Store" message reappears, and the process repeats itself.

Seeing as I'm on this forum, I am able to sign into my Apple account from the computer, but not on any apps. I would greatly appreciate some help with this, thanks in advance.