How to search as for specific tag on data from came HTML?

I am a beginner web developer. I am trying to develop a website. It looks like : https://sketchtoy.com/69109818

I will select a category (box's are a category sorry for my drawing :D) and before selection category that will select is class="item" after selecting selected category's class="item active", i will write what for i am searching than search. Later, website will come to me data from my json file connected to my category. (I will categorize my json data as for category boxes)

How can i do this? What should I learn? , i finished frond-end design and i am learning node.js for back-end server programming.

Especially, my question is "How can i use class="item active" for categorize my json data?