Google Analytics is my preferred method for monitoring visitors to your website. To set it up:
<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script type="text/javascript">
try {
var pageTracker = _gat._getTracker("UA-YOUR-GOOGLE-ID");
pageTracker._trackPageview();
} catch(err) {}</script>
Add the markup just before the </body> tag to the footer.php file in your theme's templete directory e.g. if you are using the classic template, add it to wp-content/themes/classic/footer.php
Add the markup above to the file lib/tpl/defaul/main.php before </body> tag