The first thing we need to start exploring Solr is a working Java installation. If you already have one (for example, if you are a Java programmer you will probably have it), you can skip to the Installing and testing Solr section directly.
If you don't have Java installed, please download the most appropriate version for your operating system from http://www.java.com/it/download/manual.jsp, paying attention to the appropriate architecture choice (32 or 64 bit) as well. Since it's impossible to provide a detailed step-by-step description on how to install Java for every platform, I'll ask you to follow the steps described on the Oracle site: http://www.java.com/en/download/help/download_options.xml. The described procedure is not complex; it would require only a few minutes.
Note
The Java Virtual Machine (JVM) has been open sourced since some years, so it's possible to use some alternative implementation of the JVM specification. For example, there is an OpenJDK implementation for the *nix users (http://openjdk.java.net/), or the IBM one (http://www.ibm.com/developerworks/java/jdk/); these are largely adopted, but we will use the official Java distribution from Oracle. The official documentation warns about GNU's GCJ, which is not supported and does not work with Solr.
When Java is installed, we need to configure two environment variables: PATH and CLASSPATH. These are described again at the Oracle website: http://docs.oracle.com/javase/tutorial/essential/environment/paths.html.
The PATH variable is generally used to make a command available on the terminal, without having to prepend the full path to call it; basically we will be able to call the Java interpreter from within every folder. For example, the simplest way to verify that this variable is correctly configured is to ask for the Java version installed, which is done as follows:
>> java -version
The CLASSPATH variable is needed to load the core Java libraries instead and makes them available to Solr components.