If you are from a qualified country, follow these steps to activate your free trial of PlayStation Plus:
Log in to the PlayStation Plus website for your region.
Click Join PS Plus at the top right or Renew PS Plus if you do not have an account.
Choose a plan that suits you.
On the following tab, click Add to Cart.
Make a new profile or sign up with one you already have.
Perform security checks if you are prompted to make sure you are not a robot.
Fill in the details for your credit or debit card.
Click Finish to confirm the details and start the free trial.