No problem, and though I know it may sound daunting, it’s a lot easier to do than it is to explain.
The “coming soon” thing is exceptionally easy, and requires no additional domain or redirection:
Just place a “coming soon” page in an index.html file, and place it in the directory that contains the Joomla! install.
The server will find the index.html file and serve it when someone browses to the page. When you are ready to display your Joomla! site, just delete/rename index.html, and the Joomla! site will load from the index.php site that drives it. As index.html is found before index.php, this is an easy way to “mask” a developing site.