You'll need to first set the SMTP settings for your host - Administration > vCenter Server Settings > Mail.
Then for a specific alarm you can do the following:
1) Select the Alarm definitions screen, right click on the alarm and select Edit Settings. If you only see View Settings then you need to select the vCenter object upon which the alarm has been defined.
2) On the Actions tab for the Alarm, click Add to create a new action. The default option is Send an email notification. In the Configuration field enter an email adderss. Then select Once or Repeat in the columns Green > Yellow, Yellow > Red, Red > Yellow and Yellow > Green as required in your environment.
See the online documenation for more details.