Some themes have the option to add it without having to change anything, check your themes options first. Failing that, you can also do it with the following plugin; http://wordpress.org/extend/plugins/google-analytics-for-wordpress/ If you want to edit your theme yourself; http://www.youtube.com/watch?v=RjNYRGR_KWY
Using the default theme with wordpress elaves you 2 choices - add it through a plugin or modify the theme, both of which you can do with; Failing that, you can also do it with the following plugin; http://wordpress.org/extend/plugins/...for-wordpress/ If you want to edit your theme yourself; http://www.youtube.com/watch?v=RjNYRGR_KWY
Sorry, the link for the plugin didnt paste correctly. It is here; http://wordpress.org/extend/plugins/google-analytics-for-wordpress/ Install the plugin, activate it and paste in your google analytics code. Job done.
You need to go to the plugins menu, and activate it first. Then, you need to configure the settings. Go to the settings menu and click on "google analytics". I think you just need to paste in your google analytics ID and you can change whatever settings you want on that page.
use plugin google analytics for wp or enter code manually > appearance > theme editor > on right hand side find footer.php file and before </body> tag paste ur code there.