Before you officially pick a greeting, why not practice on making your voice even and understandable? I know that my voice rises when I speak on the phone, and I tend to speak faster, so I practice in the mirror to slow down my voice and make sure that I am speaking as naturally as I can.
If you've forgotten your retrieval number, call 800-244-1111 to get it. Alternately, the number was included in the welcome letter you received when you signed up for voicemail service.

Set up Visual Voicemail. If your carrier supports Visual Voicemail, you can set it up with these steps: Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now. Create a voicemail password, then tap Done. Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can record a new greeting.
Android phones can access voicemail by holding down the 1 key on the dial pad until the phone dials your 10-digit phone number. You will automatically connect to your mailbox and be prompted to enter your temporary password; this password is the last 4 digits of your phone number followed by the # key. Start the Google Voice app. Tap the three horizontal lines at the top left of the screen (sometimes called a hamburger menu), and then tap “Settings.” In the Voicemail section, tap “Voicemail greeting.” Tap “Record a greeting.” How do I access my voicemail? To call your voicemail on an Android phone, simply open your phone’s dial pad and hold your finger down on the “1” key. You can also call your voicemail from a different phone by calling your own number and tapping the pound key. How do you check your voicemail?