If you cancel your Prime membership before the trial period ends, then you will not be charged for any Prime membership fees. If you cancel after the trial period ends, then you will be charged the full price of the Prime membership fee.
After 30 days, the Prime membership will automatically renew at a price of $99 per year.
When you sign up for a free trial, the company will usually send you an email with a link to cancel. If they don’t, then check your account’s main page for a link that says “cancel my free trial.