To compose a new email message click on the 
button on the Main Window or select Message 
Compose from the menu.
A new message can also be started from the address books, by selecting an address, and either using the context-menu (right-click), or pressing the Compose button.
However a message is started, its properties (To: address, From: address, Real Name, Signature etc) will be taken from the Folder Properties of the currently-selected folder.
You can compose in Text or HTML format, depending on how you have configured MailCOPA in the Preferences or Folder Properties.
Please see the pages on Plain Text or HTML editors for more detail.