Experience trumps certifications. However, if you don't have the experience, I believe certification improves marketability.
I wanted to get into Hadoop development. I started with attending Cloudera's Apache Hadoop Developer training, a 4 day course with a good overview and some exercises.
I then spent a month reading Hadoop the definitive guide (3rd edition), writing some programs in a Hadoop environment I set up at home. I cleared the Apache Hadoop Developer certification, last week (April 30, 2013). For more information on the certification track - visit http://university.cloudera.com/certification/CCDH/CCD-410.html.
The exam is not something that one can pass without understanding Hadoop. Some questions were tricky, with multiple answers that were very close. If you had to spend on just one book, Hadoop the definitive guide, 3rd edition, along with the documentation links that Cloudera has provided will be sufficient to clear the exam.
Good luck, if you are planning to get certified!