Query to Secondary on Sharded Cluster How to specify query from mongos shell which is inteded to query data from secondary?

The goal is not to bother primary load on write and use data in secondary shards instead.
You have two options:

1) The safest is to use on your driver the "secondary" option as described on the below article:

http://docs.mongodb.org/manual/reference/read-preference/

2) Another option is to tag your secondaries as described below and target your queries against the tag, most drivers offer that option. 

Disadvantage is that you have to ensure that these nodes can't become primaries else you will target primaries. Advantage is that you can target certain operations to certain nodes.

http://docs.mongodb.org/manual/tutorial/configure-replica-set-tag-sets/#add-tag-sets-to-a-replica-set

I would say to stick with first option if you just starting using mongo.