I do not think every bot/crawler obeys "Robots.txt". So, it may not be possible to block all bots. However if you want to block Google/Yahoo/Bing and some others, you need to edit your "Robots.txt" to instruct these bots.
If you do not have "Robots.txt" in the home folder of your website, create a file "Robots.txt" there. If you already have "Robots.txt", edit the file to add the following lines:
User-agent: *
Disallow: /
