You do not have to contribute anything.
Your employer will pay an age-related contribution and if you do contribute, your employer will pay an additional amount to match your contributions up to 3% of your pensionable earnings.
Therefore, if you decide to contribute 2% of your pensionable earnings, your employer will pay an extra 2% of your pensionable earnings on top of the age-related contribution. If you decided to contribute 5%, your employer would pay an extra 3%, as the extra matching contributions are limited to 3% of your pensionable earnings. Your contributions will be based on your pensionable earnings, so if you are receiving reduced pay, you will only pay contributions on the pay you actually receive.