You can read more about how to build Infinispan in the documentation:
http://infinispan.org/docs/dev/contributing/contributing.html
Provided you already have Java and Maven installed, you can get started right away. For convenience you can use the provided maven-settings.xml file which enables all additional repositories required for building Infinispan:
mvn -s maven-settings.xml clean install
or use the provided build.sh or build.bat depending on your platform of choice.