By default, mod_deflate module is enabled in Apache. To make sure that it is enabled, check following line in Apache configuration file.
LoadModule deflate_module modules/mod_deflate.so
Now edit your apache configuration and add the extensions you want to compress.
vim /etc/httpd/conf/httpd.conf
Add the following lines at end of the file
AddOutputFilterByType DEFLATE text/plain AddOutputFilterByType DEFLATE text/html AddOutputFilterByType DEFLATE text/xml AddOutputFilterByType DEFLATE text/css AddOutputFilterByType DEFLATE text/js AddOutputFilterByType DEFLATE text/javascript AddOutputFilterByType DEFLATE text/javascript-x AddOutputFilterByType DEFLATE application/xml AddOutputFilterByType DEFLATE application/xhtml+xml AddOutputFilterByType DEFLATE application/rss+xml AddOutputFilterByType DEFLATE application/javascript AddOutputFilterByType DEFLATE application/x-javascript
Restart Apache Service