Use the template tag, the_author().
The WordPress Default theme has an example of the_author in the wp-content/themes/default/index.php but the code is ‘commented-out’ so it doesn’t execute. So if you wanted to display the author info with the WordPress Default theme you would change the line in wp-content/theme/default/index.php
from:
<small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
to
<small><?php the_time('F jS, Y') ?> by <?php the_author() ?> </small>
This also demonstrates the_time function.
Related:
Stepping Into Template Tags
Stepping Into Templates
Template Hierarchy
