Making the First Post Full Width

I am wanting to make my first post full width while the rest of the posts are aligned left with the sidebar on the right. (for example: https://galmeetsglam.com). How would I go about doing this? I have been searching but nothing I’ve found has worked.

I have asked the same question to the support team for my template but they have not come up with any answers for me.

My site is here: https://www.lovesarahblog.com

Thank you!