Where to insert data after sharding?
With a sharded cluster, you need to insert data via a mongos process. A mongos process acts as a query router: it directs queries/updates to the appropriate shard(s) based on the current cluster configuration and metadata.  Typically you deploy one mongos per application server.

From your driver point of view, you can update your connect string and replace the address of your standalone server or replica set seed lists with one or more mongos servers to use.

For more information, you're best starting with the Sharding Introduction in the MongoDB manual.