Got a tracking code that you want to paste into every page of your site? The meta_tags file is a great place to do this. It's in your Snippets folder.
If you need help finding your Theme files, you can follow the directions here: Custom Themes.
Just place the code under the Site-wide meta tags,
Don't forget to save your work and make a note of the change you've made in case you need to find it later.