There's quite a few information available on how to install Apache Solr for your Drupal website. One of the best places to start is the Apache Solr Search Integration module documentation page. In this post I will gather all the bits and pieces for installing Solr in Tomcat on one specific platform: Snow Leopard. This is the platform I'm developing Drupal sites on and the great thing is it has all the needed Java stuff built in, so it's quite easy to install Solr and Tomcat. This method might work on some other systems too having Java 1.6 (with mostly some minor adjustments) but I've not tested this.