How to change the database mode to STANDBY?
I Thought that I can change the DatabaseLS_Main from Standby mode to bring it online, take the snapshot and bring it back to STANDBY mode...is it right to do ?

What you are telling is not possible using logshipping.
There are limitations of database snapshots:

In a log shipping configuration, database snapshots can be created only on the primary database, not on a secondary database. If you switch roles between the primary server instance and a secondary server instance, you must drop all the database snapshots before you can set the primary database up as a secondary database.

There are some options:

If your database is not that big, you can do a snapshot replication, if you really want a snapshot of your database as opposed to doing a backup/restore.
If you want a near-to-realtime data on some secondary server, then transactional replication would be a better option.
Alternatively, you can use database mirroring. It allows database snapshot on the mirrored server but the database must be in the synchronized mirroring state.
Now that SQL Server 2012 and 2014 are out with AlwaysON, you can consider that as well if you want a read-only copy of your database.

Note that database snapshots as well as AlwaysON are Enterprise features :-)