You can start a cryptocurrency with only airdrop same as DeepOnion and Privcy but those will not be huge successful crypto. On the other hand, you can launch a project with 0 premine and let it mine by the community or people like the GRIN coin.
Another question is let say the price have a ratio during ICO, 1 BTC = 100 CRYPTO, so the price per CRYPTO = 0.01BTC, so when they enter exchange, the price will start as 0.01BTC?
But what if the CRYPTO doesnt start with an ICO, how the price going to set to launch?
No one is going to set the price, only buyer and seller will do it. For example, your coin is listed in Binance and one buyer bid 0.001 BTC per coin and someone also wants to sell in this price, only then your coin price will have a start. In a word, until having a trade of your coin, no price is there. However, the bid price will be the initial price.