The first step is to turn off the option to have your site indexed by search engines or to make you site private. These options are part of your privacy settings, which we explain how to adjust here.
Once you have updated these settings, you may still need to contact Google and ask them to re-index your site to remove it from searches. To do that, click here and follow the step-by-step instructions.
Note that if you have turned off the option to have your site indexed and then changed your mind and turned it back on, it may take several weeks (at least) before Google's indexes will recognize your site as being findable.