Well, hadoop is actually a framework that has lots of component. In general, python or Java us preferred for writing MapReduce program. So, you need get familarize with java or python at first.
Next, you need to setup hadoop in your local pc. For this you can download the cloudera quickstart VM which has all the services already installed in it. Here is the link to download the quickstart VM:
Finally, you need to understand the overall architecture of Hadoop and what are different components in it. For this you will find various resources on the internet or what you can do is you can refer to O'reilly's book called as Hadoop - The Definitive Guide. To be honest it is a the bible for hadoop. You will find very good explanation and good examples in the book. All you have to do is follow along.