overwriting previous code

Hi-I’ve been using WP to manage a site that was built in html/php but decided I wanted to dump it in exchange for a new WP theme. Theme is installed but is coming up with these errors.
Warning: Cannot modify header information – headers already sent by (output started at /home/content/35/7777935/html/wp-includes/general-template.php:2102) in /home/content/35/7777935/html/wp-includes/option.php on line 568

Warning: Cannot modify header information – headers already sent by (output started at /home/content/35/7777935/html/wp-includes/general-template.php:2102) in /home/content/35/7777935/html/wp-includes/option.php on line 569

How can I wipe the old code and content and start fresh?

THANK YOU!!!!