First, you'll want to add this line of code between the <head> and </head> sections of your site:
<!-- This includes the latest jQuery build -->
The above code will include the latest build of jQuery into the page you're working on. You may chose to host the jQuery library locally, if you want to do that then simply visit the official jQuery site, download a copy and then upload it to your site (I recommend putting it into a folder called 'js').
If you've decided to host it locally, the code between your <head> and </head> sections should be this instead:
<!-- include jQuery library -->
After you have the jQuery library included in your page, you can then use the code that's been provided above by smajer. Note that the <script> code should be placed again between the <head> and </head> tags to ensure it's loaded before the site is.