See http://wiki.apache.org/httpd/DistrosDefaultLayout for discussion of where you might find Apache httpd configuration files on various platforms, since this can vary from release to release and platform to platform. The most common answer, however, is either /etc/apache/conf or /etc/httpd/conf
Generically, you can determine the answer by running the command:
(That's a capital V). Or, on systems where httpd is renamed, perhaps
This will return various details about how httpd is built and configured, including the default location of the main configuration file.
One of the lines of output should look like:
which, combined with the line:
will give you a full path to the default location of the configuration file