I want first Button to be in a focused state on page load

I hope there is a fast way to solve this i just wanted to know on my websites mattress page i have 3 buttons each is a different mattress and when you select one it shows you the mattress image and the layers down below i have everything working however when the page loads or you go to the mattress page i want the first button (option) to be focused (selected) by default on page load. Can someone help me with that?

I have also attached an image as to how i would like when the page loads!

Here is the page link link: Mattress