New accounts that have less than 2000 subscribers have the Freemium payment plan. You can change the plan to monthly subscription or to pay as you go (credits) any time you want, or when the contact list size exceeds 2000 subscribers.
Monthly subscription - you can send unlimited number of emails to your subscribers every month. The monthly price depends on the number of active contacts you have in your database, you can calculate your monthly price here.
Pay as you go - every email is 1 point and you pay only when you want to send emails. For example, if you have 5000 subscribers, you need to order 5000 credit points to send them all an email. Pay as you go plan is reasonable if you are not sending emails to your subscribers every month. Credit points expire after one year. More info about the credit prices can be found from here.
To choose or change your account plan, click on the current plan name in the upper right corner of the window.
The other option is to click on the upper right corner on your name and then from the dropdown menu on "Preferences".
Your account preferences window appears, where you can choose the "billing preferences" tab. There you can choose the subscription plan you want (1), fill in the billing information (2) and then click on "Save changes" (3). Your payment plan will be changed.