I want to start mining new blockchain (exactly as Satoshi did in 2009) for testing purpose I will mine myself ONLY so difficulty will be always in 1 range so CPU is enough for this task

So my question is: how to setup bitcoin core 0.12 to start mining with CPU new blockchain ?

If I type in the console: setgenerate true and wait a few hours it's possible to mine first block with 50 BTC (of course with internet connection OFF to prevent download blocks)?

I know that this BTC will be worthless ATM (without $ value) but I want to do that anyway to better understand mining process
First, create a directory to store your blocks and configuration:

$ mkdir mybitcoindir
$ cd mybitcoindir


next, you need to create a conf file called bitcoin.conf that has this in it:

checkpoints=0
connect=0.0.0.0
daemon=1
discover=0
dnsseed=0
gen=1
irc=0
keypool=5
listen=0
port=19002
rpcpassword=test123
rpcport=19003
rpcuser=test
upnp=0


Now start up your bitcoin daemon

$ bitcoind -datadir=mybitcoindir


You should have no outbound connections

$ bitcoin-cli -datadir=mybitcoindir getpeerinfo
[
]


Now you can generate coins

$ bitcoin-cli -datadir=mybitcoindir setgenerate true


Note even with the difficulty at 1, generating coins off of a single CPU can take a while. Expect it to take a few hours to find your first block. You're better off getting a really cheap USB miner as they can find about 2-3 blocks at difficulty 1 every minute. But that, of course, requires a bunch of setup as well.

Best of luck.