You write what you want to write, then highlight (select) the text you want to edit, then you click on the button of the editing you want to do, like bold for example so you click on the
B button, this will insert things called tags between brackets before and after the selected text.
Repeat the step as necessary for anything you want, like font color, italic, underlined, and the size.
Those tags will change the way your post look like AFTER posting, not during writing it, you can post it and see or you can click "Preview Posts" to see that those 'rubbish letters' have altered the way your post looks.
MAKE SURE YOU WRITE FIRST => SELECT => CLICK ON EDIT BUTTONS.
Hope that was clear enough...
Take Care