help: error on adding categories

hello, I’m new to WP and so far I’m 50% finished with the lay-out and coding..but I can’t seem to get my widgets to show on my sidebar..I can see the widgets on myadmin but I can’t see it on my site.

also when I try to add categories I keep getting this error (“Warning: Cannot modify header information – headers already sent by (output started at /home/MY-SITE/public_html/MY-SUB/wp-includes/functions.php:1517) in /home/MY-SITE/public_html/MY-SUB/wp-includes/classes.php on line 734”)

I have no idea what that error means T_T

Thanks in advance..