How to add featured image to a new wordpress theme

1) Add featured image setting on functions.php

function theme_post_thumbnails() {
add_theme_support( 'post-thumbnails' );
}

add_action( 'after_setup_theme', 'theme_post_thumbnails' );

2) Add the feature image to your post

3) Add the image to your single posts

Here you have to go to single.php, or create that file in the case that you don't have it, and here write something like this:

4) Now go to your post, and... thats all!

Sayonara