Create a page to be your front page, then on the reading settings page select use a static front page then choose your page from the dropdown.
EDIT:
A quick and dirty hack based on your comment would be to convert your static HTML page to PHP and use that as a template. You can easily convert your HTML file to PHP by changing the file extenstion to php.
So landing.html becomes landing.php.
Next add the following code to the very top of that file:
<?php /* Template Name: Temp Landing Page */ ?>
Pop the file in your theme folder along with any images etc. wp_content\themes\yourtheme
In the admin area, edit your page and on the right there should be a dropdown called template. Select the temporary template out of that list and hit save.
This method is not recommended at all for production sites but should be enough to get you up and running.
EDIT 2:
The long term, “correct” option would be to set up a subdomain called http://test.yourdomain.com or http://dev.yourdomain.com and install another instance of WordPress on there while you practice and develop your site. You could then restrict access via the htaccess file based on IP. This method is dependent on your hosting options, you might be restricted on the number of databases. You could install your dev site to the same database with a different prefix dev_.
Alternatively you could install a web server on your local computer and host your dev site there. It would only be accessible to you but it is less risky than having a publicly accessible dev site.
I use a combination of the two for my sites.
- A local web server for immediate debugging and coding.
- A remote test domain for demoing the functionality and stability testing
- A live site that I only apply code to when both me and the client are happy with the test one.
There are several web server packages available for local hosting, I use XAMPP on both my Mac and Windows PC. Dead easy to set up and low maintenance.