Joomla has a very nice feature that allows you to test your site in complete privacy. You might just need your site offline for a few minutes, you might want to show it privately to a client, or you might want to keep the site offline for weeks while you build it. Either way, this is a good solution.
Login To Your Joomla Site

Go to Global Configuration

Click on Site then Global Configuration
Site Offline

Set "Site Offline" to "Yes" and then click Save
Visit Your Site

Now when you visit the front of your site you'll see a screen like this. Anyone who has a Super Administrator, Administrator or Manager account can login and view the site as normal. Everyone else will only be able to see the splash. You can keep search engines, rivals and all other unwanted guests out of your site until you're ready.
If you'd like to take this one step further and customize your offline page with a new logo and text, click here for the next tutorial.