In the Newsweek article on blogs, there was an anecdote about a teenage boy whose mom found his blog and wasn't happy about all of what she read in it. This illustrates an important lesson. Unless you password protect it, anything that you put up on the web is potentially visible to everybody. You may not tell anybody about your URL, but given a month or so, Google will find you. And then a perfectly random search query could turn up your site.
If you really want to keep it private, you need to use password protection. This will prevent Google or other search engines from finding you and even if someone does get your URL somehow, they will need to know a valid username and password to actually view the page. A free site that allows you to create a password-protected blog or diary is Diaryland. If you have your own webspace you may be able to use something called an .htaccess file to create your own password-protected directories (you can also do lots of other cool things with .htaccess files). Here's a good introduction to .htaccess files if you'd like to learn more.