1) Is a good moment for start to mine?
2) I have a good pc with 2x GTX Asus 1070 and a GTX 750ti, can i mine good or not?
You will not be able to mine Bitcoin. The difficulty of Bitcoin mining is too high for CPU or GPU mining. You need to use specialized mining ASICs in order to actually mine a substantial amount of Bitcoin. To even make a profit, you also need very cheap electrcity.
3) I need to start from some altcoin and trade for bitcoin?
If you actually want to do GPU mining, you need to mine altcoins, not Bitcoin.