Tip: mouse over any button to see its functionality and associated keyboard shortcut. The last button on the right ("Remove Formatting") is there to clean up colors, fonts, images, etc. from text you pasted from another source, like a Microsoft Word document (lots of specific formatting).
Note: double hyphens ("
--") above your signature honor a Usenet convention from earlier times. You can't prevent Gmail from adding the double-dashes, but you can delete them.