Within your cPanel there is a feature where you are able to create custom error pages for common errors that show up, such as 404's or 500 errors.
Once logged into your cPanel under the Advanced category, click on the icon that says error pages.
On the next page you can choose which domains you'd like to create the error pages for.
You're then taken to the page with the list of error messages, pick the one you'd like to edit. I would go for the 404 and 500 error pages.
After choosing the domain and clicking on the GO button, you're taken to the "editor" page. HTML knowledge is useful for this section, but not enforced. There are also a few tags that you can use provided by cPanel.
Once you are finished, scroll to the bottom and click on the save button. You're error page will be called 404.shtml (if editing the 404 page) and you can view it, by either going to a page on your server that doesn't exist or simply putting into the address bar:
http://yourdomain.com/404.shtml
Pretty neat, huh? =)