When I connect to geth with
it automatically connects to Main Network. I would like to connect TEST-NET. How I could connect to TEST-NET with geth?
You need to launch geth console by giving "testnet" or "rinkeby" as an argument -
--testnet Ropsten network: pre-configured proof-of-work test network --rinkeby Rinkeby network: pre-configured proof-of-authority test network
There are other parameters/arguments available as well, which you may find worth taking a look at, please check this link for more details.
Also see here for the differences between the various test nets.