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 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 aren't sending emails to your subscribers every month. 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".
A new window appears with your account billing preferences, where you can choose the payment plan. Choose the plan you want (1) and then click on "Save changes" (2). Your payment plan will be changed.