When you are setting up your store, you can make it invisible to your customers.
You have two possibilities:
Set up a password
Go to 'Settings' > Click on 'General' > Click on the button next to 'Store password protection'.
Activating the Cover or Launch theme
Cover - this theme will let you create a mailing list before the official launch of your store - Check this theme out.
Launch - this theme shows the time left for the launch of your store - Check this theme out.
Install the Cover or Launch theme. Then go to 'Layout' > and under Installed themes, you should see the theme you just installed. Click on the button next to 'Activate'.
Read the article: How to install and activate a theme?
Meanwhile, you can work on your store's layout on one of your inactive themes. Learn more on how to edit inactive themes.