I want my post images to open in a fancybox lightbox. I have two wordpress websites with two different themes. I am using this code for my images in the lightbox

<div id="gallery-1" class="gallery galleryid-25830 gallery-columns-3 gallery-size-thumbnail">
<img class="attachment-thumbnail alignleft" title="image-title" src="image-url" alt="" width="150" height="150" />

This code works in one theme, but in the other theme it goes to the image's url. I checked my theme files and found that there is no jquery or js file for fancybox. So I am wondering how I can add these files to my theme and what I should change in my CSS file. Please someone, help me. I am new to coding.
I have fancybox folder in other theme, it contains some images and jquery.fancybox-1.3.4. can i use them? and if yes, then how? plz someone help