How do I get the app to speak what I have written on the Message box?

First, form the sentence that you want to speak, by tapping the words to create the sentence in the Message box. Then tap anywhere inside the Message box, to speak the message aloud. 

See screenshots below. 

For Avaz ver 6.6+

Note: In Ver 6.6+, an explicit Speak button has been added to speak the message aloud. This is in addition to tapping anywhere on the message box to speak the content aloud. 

For Avaz versions less than ver 6.6

Sometimes the audio may not work as expected. Please see the following link to troubleshoot the problem. 

The audio is not working - what do I do?

You can also set various options for speaking the audio during various stages of forming a sentence. 

Click here to read more about that - What are the various audio settings to speak aloud only words or full sentences