You can setup additional website under your existing hosting account using the following steps,

1. Please login to your cPanel.
2. Please find a section "Addon Domains" under your cPanel.
3. Please insert your newly purchased domain name in "New Domain Name:" section.
4. Press a TAB button on your keyboard & it will select a appropriate username for your domain name & document root.
5. Please go to password section and insert a password as per your requirements.
6. Finally click the button "Add Domain".

You're done.

NOTE: Please make sure you are pointing your domain name to correct name servers before adding it as an addon domain. Please use the following name servers on your domain name,

ns1.hl-servers.com
ns2.hl-servers.com
ns3.hl-servers.com