1- How to set the price of my coin?
Work out the value of what you're offering overall and then set the price to the mktcap of that. But what is it your coin is trying to do? Why would I want it?
2- How to set the total amount of coins to used?
The total number of coins available will be defined by the smart contract that produces the erc token.
3- Can I add my coin in the trading platforms?
Yeah most companies will charge a fee between around $2000+ to add your coin to their exchange afaik.