To do that go to the gear in the upper right and click on it for the drop down settings. Click on the settings link and scroll down the page that opens until you get to the Keyboard shortcuts choices.
When done, be sure to click the Save Settings at the bottom of the page.

Once enabled a few of the email shortcuts you are most likely to use are:
- c : compose new message
- r : reply
- a : reply all
- f : forward
- Shift + r : reply in new window
- Shift + a : reply all in new window
- Shift + f : forward in a new window
You can find more in-depth details on Googles gmail help page
Finally, for this email, if you are not familiar with them all, or as a reminder, the most commonly used Keyboard Shortcuts, when working in mail or any other documents, are:
Copy, Cut, Paste, Delete, Undo, Redo (the Undo – seemingly only in Word), and Find. I love when I remember to use “Find” for word search in a document as well as on web pages. It’s certainly a lot more efficient than scrolling through a page doing an “eyeball” search.
You can see the commands in my post – 7 Keyboard Shortcuts. Not on that page is the one for highlighting ALL of the content in a document, in an email or on a web page. Be sure to place your mouse in the area you want highlighted; then click Control or Command>a.