Templates--need guidance on how to use

I am new, getting our blog started, but so confused how to set up a template. And once I have set up a template, I want to set it as the default to use for blog posts in the future–and I want to assign the blog posts I have already created to this new template. Please guide me as to where I should begin to learn how to do this. I have wasted dozens of hours scrolling through the Pressroom interface trying to figure it out and have gotten nowhere.

Thank you!