There are 2 editors in WordPress: "HTML Editor" and "Visual Editor". WordPress users can use both these editors. But we can set which editor should be displayed by default.

If you prefer to the HTML Editor, use this code (insert them into functions.php):

# set HTML Editor as the default editor
add_filter( 'wp_default_editor', create_function('', 'return "html";') );

If you prefer to the Visual Editor, use this code (insert them into functions.php):

# set Visual Editor as the default editor
add_filter( 'wp_default_editor', create_function('', 'return "tinymce";') );