i find something like "on monero booth party is encrypted so no one know until you given the watch key for SINGLE transaction"
Do you mean the view key or the transaction key? If the former, it is true regardless the transaction. If the latter, it is not used to reveal amount, simply to prove payment.
then does i will be able to know the client address like... the john send 10$ via monero
You will be able to know the transactions you made with your clients, and they will know as well, including your addresses. What you can't know is where they spend their XMR.