[Ethereum] Inspecting private variables of a contract

You cannot prevent anyone with access to the blockchain from reading state and executing code in any way they wish. You can encrypt data, but that doesn't prevent people from seeing it, just interpreting it.

Assuming you're happy with people who have stopped subscribing being able to access the data that was already available to them - something you fundamentally can't prevent anyway - there are a couple of ways you could achieve this.

One is to use standard multi-recipient public key encryption: Encrypt each message with a randomly generated key, then encrypt that key with the public key of each user who should be able to access it. When a user ceases their subscription, remove them from the set of active keys. When a new user subscribes, add them to the set, and encrypt the secrets of any past messages they should be able to access.

A slight variation may permit easier key management: Use one key for each 'epoch'. An epoch ends each time a user ends their subscription. Whenever you insert a message, encrypt it using the current epoch key (make sure to use an IV too, to prevent dictionary and known-plaintext attacks). Provide a keystore that lets users access the epoch keys for every epoch they're subscribed to; either off-chain, or by encrypting those keys with the public key of each user who's permitted to use them.

Setting up a private testnet is not necessary to commence on solidity programming. However in case you are interested in doing it, you can take a look at this blog: http://iotbl.blogspot.in/2017/03/setting-up-private-ethereum-testnet.html

You can commence solidity programming by writing a simple Dapp which fetches some hardcoded data from a SmartContract (say a SmartContract that just displays Hello World)?

If you are confused as to how to proceed with this, I would recommend watching these YoutTube videos to understand how it is done: https://www.youtube.com/watch?v=9_coM_g7Dbg&list=PLH4m2oS2ratdoHFEkGvwvd7TkeTv4sa7Z