how do i dispute a transaction

That would depend on where you saw the transaction. If you saw this in your bank or credit card statement, see this, If you see apple.com/bill, itunes.com/bill, or an unfamiliar charge on your statement - Apple Support

If all you got was an email, it is probably a phishing attempt to get your account information. See this, Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

And this, Identify legitimate emails from the App Store or iTunes Store - Apple Support