Broker's costs selling/buying?

I am thinking about to sell my condo apartment at Upper West Side. Since I'm from Europe I'm not familiar with the system. Is there anybody who can explain it to me? I have heard the broker of the seller will ask for some percentage (how much would be usual?) of the sale and if there is on the buyer’s side a broker involved the brokers share the money. And I'm also wondering if I buy a condo with the help of a broker - let say direct from a sponsor - who is paying the broker who is helping me? I guess, the sponsor is paying him, so that means at the end I'm as the buyer would pay the broker finally, because the sponsor would calculate the fee (how much usually?)for the broker in his offer. Every information would be of help. Thank you.