In the privacy settings, you can password protect your site.
This will stop anyone from entering and viewing your site freely. Only people who know your password will be able to enter your site. It will also stop search engine bots to 'crawl' your site, meaning that it won't show up in searches that users make for your site. Please note that as soon as you take the password protection off, a search engine bot could find your site within minutes.
To set up a password, go into 'Admin,' 'Setup,' then 'Privacy.'
Tick the checkbox and enter a password of your choice into the box. Then click 'Save changes.'