http://docs.moodle.org/en/Debugging
In moodle/config.php you can add the lines:
$CFG->debug = 0;
$CFG->debugdisplay = 0;
or you can use SQL commands
UPDATE mdl_config SET value = 0 WHERE name = 'debug';
UPDATE mdl_config SET value = 0 WHERE name = 'debugdisplay';
or you can use tools like phpmyadmin to edit those rows directly in database and table mdl_config
The other problems may be caused by hacked php files. Read other recent posts from this forum, security documentation or search from forums for example "hacked" - upgrade your site and check all security settings of your site.