Since you are using shared hosting (Cpanel) you need to have SSH access to your server. You can easily enable SSH for your shared hosting server. How to enable SSH/Shell Access in cPanel might help you to do so else use Google it.
Installing and configuring open fire
Login to your server via SSH in terminal
ssh username_of_server@ip_adress_of_server
Run in terminal
sudo apt-get update
sudo apt-get upgradeInstall JRE ( if you didn't have already because Openfire is in JAVA)
sudo apt-get install openjdk-7-jre
Now you have enviornment of java and openfire. Following tutorial Installing Openfire heading onwards .......
I hope it helps you to kick off.