how to split the chunks in sharding
You need to enable sharding on the database test first:

sh.enableSharding("test");


Then you need to shard the collection and pick a shard key.  Based on what you were trying to split on, that would be:

sh.shardCollection("test.zp", {"city" : 1});


A couple of notes:


"city" is probably a poor shard key unless you are expecting an even distribution of records per city and you are going to have a lot of them.  Take a look at the docs on picking a key
If you have data in the collection already, you will need to create an index on the "city" field before you can use it as a shard key.  If the collection is empty, MongoDB will do it for you automatically when you use it as a shard key.