Different Linux distributions and/or UNIX operating system, and control panels stores Apache web server configuration file at different locations. There are different ways to locate configuration file.

Apache is, by default, installed in /etc/httpd directory. But this path also depends on how apache has been compiled. Default configuration file name httpd.conf.

Using httpd command

To find the location where apache is installed and to print configuration file location, run the following command:
# httpd -v

Here is the output from my Fedora Core/RedHat (RHEL) Linux server:

Server version: Apache/2.0.54
Server built:   Jan 17 2006 06:36:00
Server's Module Magic Number: 20020903:9
Architecture:   32-bit
Server compiled with....
 -D APACHE_MPM_DIR="server/mpm/prefork"
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D HTTPD_ROOT="/etc/httpd"
 -D SUEXEC_BIN="/usr/sbin/suexec"
 -D DEFAULT_PIDLOG="logs/httpd.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_LOCKFILE="logs/accept.lock"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="conf/mime.types"
 -D SERVER_CONFIG_FILE="conf/httpd.conf"

HTTPD_ROOT points to default location /etc/httpd and line SERVER_CONFIG_FILE points to configuration file conf/httpd.conf. So the path is /etc/httpd/conf/httpd.conf

If you are using Debian Linux try apache-perl:

# apache-perl -V

cPanel/Plesk Control Panel

If you are using third party control panel, then you can find out Apache configuration file inside your /home/username/ directory or /var/www/vhosts/ directory.

Using find command

This is an old good (read as geekey) classic way to locate httpd.conf file:
# find / -name 'httpd.conf' -print

You may have to wait for few seconds/minute to search complete. Or try out locate command:
# locate httpd.conf