The "Annual" plan comes with a 7-day free trial. Tap "Start free trial" to sign up. You can use the Premium Service free for 7 days; when the trial ends, the price of the Annual plan is charged automatically.
If you cancel from Google Play during the trial, you won't be charged. See "How do I cancel (stop automatic renewal)?" for the steps.
* The free trial is available for the Annual plan only, not for the "Monthly" or "Lifetime" plans.