how do I underline a word using my email?

When you open the message composition window you should see the editing menu:

Note the set of font buttons, "B I U" The third button will underline selected letters, words, etc. The first changes to Bold and the second to Italics.

You can also use the editing shortcuts: COMMAND-B, COMMAND-I, and COMMAND-U.

Select the word(s) you want underlined and press command+U

Yes, many ways. The easiest is to highlight the word (if already typed) and use Command + U. If the word hasn't been typed yet, use the same command to start & stop underlining. In addition, Command + B = bold and Command + I = italics.