The Hadoop documentation includes the information you need to get started using Hadoop. Begin with the Single Node Setup which shows you how to set up a single-node Hadoop installation. Then move on to the Cluster Setup to learn how to set up a multi-node Hadoop installation.
The Hadoop MapReduce Documentation provides the information you need to get started writing MapReduce applications. Begin with the MapReduce Tutorial which shows you how to write MapReduce applications using Java. To write MapReduce applications in languages other than Java see Hadoop Streaming, a utility that allows you to create and run jobs with any executable as the mapper or reducer.
The HDFS Documentation provides the information you need to get started using the Hadoop Distributed File System. Begin with the HDFS Users Guide to obtain an overview of the system and then move on to the HDFS Architecture Guide for more detailed information.