If you want to change some of very long list of Liferay's properties, you have to add them to portal-ext.properites located at
Ofcourse liferay and tomcat directories may vary depending on which version of Liferay you use.
And which properties to add?
The list is very long. Liferay's makers assumed that all default values would be stored in Liferay jars (actually in portal-impl.jar, in portal.properties file). And if one want to change anything he/she should override particular property value in portal-ext.properties file.
On Liferays doc page there's an exhausting page that describes all configuration properties.
After changing entries in properties file you should restart Liferay. It's obvious.