Warning: Cannot Modify header information

This is a WordPress theme I have modified in Dreamweaver
When I post it to my domain name barbarapedersen.ca this is the message I get.
“Warning: Cannot modify header information – headers already sent by (output started at /hermes/bosweb/web123/b1233/nf.petalpower/public_html/barbarapedersen.ca/index.php:2) in /hermes/bosweb/web123/b1233/nf.petalpower/public_html/barbarapedersen.ca/wp-includes/functions.php on line 3371”

this is what the line 3371 in wp-includes/functions.php is
“header( ‘Content-Type: text/html; charset=utf-8’ );”

Any comments or help is much appreciated thanks