how do i teach my dictation a new word

Dictation, provided you are using a quality microphone with distinct, and not heavily accented speech patterns, will gradually adjust to your speech pattern, but is not accumulating the words that you speak.

Using System Preferences : Accessibility : Dictation : Dictation Commands... panel, you can tell Dictation to paste a certain word, or phrase when you speak something. For instance, I had a problem with enhanced dictation understanding the proper noun "Nancy." In the mentioned panel, I added a new command as shown below:

When you tell Dictation to “show commands,” a User section will appear with these customizations. The “Show Extension” will allow me to select a file in the Finder whose extension is hidden, and speaking those words, unhide the extension by running an AppleScript.