The initial price of the ICO is set by the owner so it‘s up to you to set a fixed price per token. Most of the ICOs prefer a simple price like $0.1 per token.
You choose the rules. If you want to increment the price per token for the investors that came later, you can. Some companies decide to charge more per token for the last tokens because it’s a way to incentivize early investors. However I believe that it’s better to just set a fixed price for everybody. That will be better for all the investors.
Once you complete the ICO, you’ll have to manually reach to all types of exchanges like poliniex, kraken, bittrex, etherdelta to list your token on those exchanges and make your token tradable.
That means that people will be able to sell and buy tokens from other users. It’s a difficult process because there are a lot of people that want to get listed on the most famous exchanges.