To read your email online, type http://www.yourdomain.com/webmail into the browser address window. Input your FULL email address for the user name (emailName@yourDomain.com) and your password for that email account.
To access Webmail from within your CPanel control panel:
Click on the Mail icon.
Click on the Add/Remove Accounts link
Click on the Read Webmail link, next to the account that you want to read.