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.
A new message can also be started by double-clicking a folder.
Unless started from an address book, a message's 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, and you can always change from one to the other from the listbox at the bottom of the Compose window.