How to set up a live test site?

Question

This is probably really obvious to most of you but i am new to this.

How do you set up a live test site that does not use the primary domain name? At the moment the primary domain name is being used for a temporary landing page and I want that to be the case until the site goes live. I am using the filezilla.

The advice I had been given was to publish it to a sub folder or the htaccess file on the server so it can be configured to restrict access (e.g. by username and password) but I am not sure how to do that. Can you let me know the steps involved, bearing in mind I do not want a second domain but to test the site on a live server so others can see it but not using the primary domain.

Many thanks,