There's some fairly limited documentation here.  You have to click on the 'Create, edit, and retrieve passwords on the command line' or 'lpass subcommands' drop downs on that web page.
A full manual page with examples is available on the Lastpass github page.
To retrieve the password for a specific website use:
lpass show --password sitename
You can also add the -c option to clip the password to your copy buffer.
Retrieving the email account for a website isn't as clear, as it depends upon in which field it's stored, if the email address is your username then use:
lpass show --username sitename
Otherwise using the correct field in place of --username should work, --id or --name may be good choices, or you can specify --field=FIELD. Failing that, look at all of the stored fields using:
lpass show --all sitename
Additionally you can view all of your stored information using:
lpass ls