1. Log into cPanel (yourdomain.com/cpanel - use your actual domain name)
2. Click on the "Mail" icon and on the next page click on "Add/Remove Accounts"
3. Find and click the "Add Account" link (usually towards the bottom of this page)
Follow the instructions on that page to create your new email account.
Please note that your email accounts will consume your main website's webspace qouta.