To subscribe a plan, navigate to the Billing tab in the Admin module. You should have a Subscription sub tab that provides a comparison of the plan features and unit costs. You can view the total cost for a plan by clicking the Select button and viewing the estimated total cost on the bottom bar.
Your selected plan for which you are viewing the costs for will read Selected. You can change between Monthly and Annual estimates by clicking on the desired timeframe.
When you are ready to sign up for a plan, click Select on your desired subscription. From there, hit Get Plan to be taken to the next payment steps.