to format your hadoop namenode.

gistfile1.txt

/usr/bin/hadoop namenode -format
11/12/31 06:28:39 INFO namenode.NameNode: STARTUP_MSG: 
/************************************************************
STARTUP_MSG: Starting NameNode
STARTUP_MSG:   host = ip-10-146-3-18.ap-northeast-1.compute.internal/10.146.3.18
STARTUP_MSG:   args = [-format]
STARTUP_MSG:   version = 0.20.2-cdh3u2
STARTUP_MSG:   build = file:///tmp/topdir/BUILD/hadoop-0.20.2-cdh3u2 -r 95a824e4005b2a94fe1c11f1ef9db4c672ba43cb; compiled by 'root' on Thu Oct 13 21:51:41 PDT 2011
************************************************************/
11/12/31 06:28:39 INFO util.GSet: VM type       = 32-bit
11/12/31 06:28:39 INFO util.GSet: 2% max memory = 19.33375 MB
11/12/31 06:28:39 INFO util.GSet: capacity      = 2^22 = 4194304 entries
11/12/31 06:28:39 INFO util.GSet: recommended=4194304, actual=4194304
11/12/31 06:28:40 INFO namenode.FSNamesystem: fsOwner=hduser
11/12/31 06:28:40 INFO namenode.FSNamesystem: supergroup=supergroup
11/12/31 06:28:40 INFO namenode.FSNamesystem: isPermissionEnabled=true
11/12/31 06:28:40 INFO namenode.FSNamesystem: dfs.block.invalidate.limit=1000
11/12/31 06:28:40 INFO namenode.FSNamesystem: isAccessTokenEnabled=false accessKeyUpdateInterval=0 min(s), accessTokenLifetime=0 min(s)
11/12/31 06:28:40 INFO common.Storage: Image file of size 112 saved in 0 seconds.
11/12/31 06:28:40 INFO common.Storage: Storage directory /tmp/hadoop-hduser/dfs/name has been successfully formatted.
11/12/31 06:28:40 INFO namenode.NameNode: SHUTDOWN_MSG: 
/************************************************************
SHUTDOWN_MSG: Shutting down NameNode at ip-10-146-3-18.ap-northeast-1.compute.internal/10.146.3.18
************************************************************/

refer to http://www.michael-noll.com/tutorials/running-hadoop-on-ubuntu-linux-single-node-cluster/