Once you have some subscribers added to your account you can start sending messages.
To send a message, go to the "Send a Message" tab.
Select the following:
Your desired recipients for your message
What date/time you'd like the message to go out
Whether you'd like your message to be sent as SMS ("regular" text) or MMS (picture message, which allows for more characters and counts as 3 message credits each)
Then scroll down and type out your message content and tap "Send."
You'll then be able to view your sent message from the "Sent Messages Report" tab: