You can use the webmail feature of your account by going to http://www.yourdomain.com/webmail/ (replace 'yourdomain.com' with your actual domain name) and logging in with the username and password for the pop3 account you wish to use.
 
If the email account is the default account you will use the following login (shown in your welcome email):
 
Username: main account username
Password: main account password
 
If the email address is an additional one setup from your cPanel, the login is as follows:
 
Username: email@domain.com
Password: password for that pop3 account
 
Replace 'email@domain.com' with the full email address you wish to access and the password should be the one you entered when setting up the email account (not your main hosting account password).
 
When you are logged in, there may be a selection of different webmail software to use.  We recommend using Horde as this has the most features and is more user friendly.