Please follow these instructions to remove it from single page:
1. open single.php
2. find and remove this line:
Posted by <?php the_author_posts_link(); ?> on <?php echo get_the_date(‘l, F jS, Y @ g:iA’); ?><br>
***
Please follow these instructions to remove it from blog page:
1. open page-blog-wide.php
2. find and remove this line (if you want to remove both author and list of categories):
<div class="newsInfo"><strong>Posted by</strong> <?php the_author_posts_link(); ?> <strong>under:</strong> <?php the_category(', '); ?> </div>
or (if you want to keep list of categories in this line):
<div class="newsInfo"><strong>Categories:</strong> <?php the_category(', '); ?> </div>