Hi, I'm guessing you're a new customer? If so, you need to register on My Verizon, and then once you've logged in there, your account number should be readily visible.
On a desktop or laptop, log into your My Verizon account and go to Account->Account Settings->Account Overview to see your account number on the website.
On the My Verizon mobile app select Account, sign in and select Edit profile & settings to see your account number.
~Jorge
