To subscribe, you can login via the website (not the app) and click on the “Subscribe” button. The current rate is $9.95/month.
You can subscribe while you are still in your trial period and you won’t be charged until it is over. If you get extensions on your trial (say by recommending the service to someone), then you won’t be charged until those extensions are over as well.
You can unsubscribe at any time by going to the billing page (more details here).