wordpress blog posts’s time

In your template where you see the_title(), you will want to change it to something along the following:

echo get_the_title() . ', ' .
    'posted by ' . 
    get_the_author() . ', ' .
    human_time_diff( get_the_time( 'U' ), current_time( 'timestamp' ) ) . 
    ' ago';

human_time_diff() is the one doing the part regarding your topic. Though you also asked for the others.

However, if you want it to look even cleaner, try:

printf(
    '<h2>%s, posted by %s, %s ago</h2>',
    get_the_title(),
    get_the_author(),
    human_time_diff( 
        get_the_time( 'U' ), 
        current_time( 'timestamp' )
    )
);

Replacing whatever tag/formatting you want in the first parameter