You should have lines similar to this in your httpd.conf:
LoadModule proxy_module modules/mod_proxy.so
LoadModule proxy_balancer_module modules/mod_proxy_balancer.so
LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
LoadModule proxy_http_module modules/mod_proxy_http.so
LoadModule proxy_ajp_module modules/mod_proxy_ajp.so
LoadModule proxy_connect_module modules/mod_proxy_connect.so
Which enable the modules in question. With the module enabled, you'll want to add configuration on how you're wanting to use mod_proxy. There's example configurations located here. Be sure to also follow the recommendations in the security area.
<Proxy *>
Order Deny,Allow
Deny from all
Allow from 192.168.0
</Proxy>
If you're configuring a reverse proxy, it's recommended to implement the paths in the relevant virtual host code blocK.
ProxyPass /foo http://foo.example.com/bar
ProxyPassReverse /foo http://foo.example.com/bar