Please check this link if you want to create new page in WHMCS http://wiki.whmcs.com/Creating_Pages
Now if you want your client to login first before accessing the newly created page, you have to add some codes below.
In your new php file (e.g. aboutus.php), you can edit the line:
if ($_SESSION['uid']) {
# User is Logged In - put any code you like here
}
and make it like this:
if ($_SESSION['uid']) {
# User is Logged In - put any code you like here
}else{
$goto="aboutus";
include("login.php");
}
Now reload the new page in your browser and it should require you to login before viewing the page.