The jQuery library is loaded by default by WordPress, so you don’t need to load it again.
If you want to add a script that makes use of the jQuery library, the cleanest is way described in the comment found in the editor when you click on the ‘Add JS Code’:
/* Add your JavaScript code here.
If you are using the jQuery library, then don't forget to wrap your code inside jQuery.ready() as follows:
jQuery(document).ready(function( $ ){
// Your code in here
});
End of comment */
As an example, if you want to make an alert with the text found in the H1 tag, then the code will be:
jQuery(document).ready(function( $ ){
alert($('h1').text());
});