Change default post style to columns

There are multiple ways to that but what I like to do is to create shortcodes e.g [one_half] :

function one_half_sh( $atts, $content = null ) {
return '<div class="one-half">' . do_shortcode($content) . '</div>';
}
add_shortcode( 'one_half', 'one_half_sh' );

Then the CSS can look like this :

.one-half {
width: 48%;
margin: 0 4% 0 0;
float: left;
}

The second column would have the same markup but with 0 margin.
This allows to set a very flexible layout you can customize on each post.

EDIT: for the second column just create another shortcode with exactly the same code but add a CSS class such as .second-col you will style in your stylesheet