CSS Help for Wordpress Blog Customizing Menu

Hello, I am starting a blog on wordpress. My theme doesn't support customizing the menu (navigation bar), but I would like to make it so that I can customize it. I did some editing to the code, but am no expert and although it now allows me to customize the menu, I get this error "Warning: Cannot modify header information - headers already sent by (output started at /home/egajones/public_html/wp-content/themes/simple-type/[login to view URL]) in /home/egajones/public_html/wp-includes/[login to view URL] on line 897" I think this can be resolved quickly if someone a bit more tech savvy than me takes a look at it!