As you can find out here, you can have custom javascript to load on every page.
Because almost every site is unique, or the theme of your CMS is a bit different, you might need to do small adjustments, without a developer to make changes to the site.
Here is a good approach with a custom SDK method to add the script when page is ready:
object.tasks.addReadyListener(function() {
// write your code here
...
});
Example to push the cartValue value in a variable.
(note that CSS selectors in the code below are just an example, and you will need to change those)
object.tasks.addReadyListener(function() {
window._vteq.push(["setVar", "cartValue", jQuery('#cart.cartValue').text()]);
});
Example for checkAndSubscribe method to subscribe user on a page, validating first the subscription checkbox.
object.tasks.addReadyListener(function() {
jQuery('#wpmc-next').on('click', function(){
object.Helpers.checkAndSubscribe('#billing_email', '#mc_newsletter', true);
});
}, 'runjs');