Waiting for operator

I want to keep waiting my incoming calls to an extension in a line, means the operator is busy and now somebody else wants to talk to him. so the system say a message and keep him waiting. and like this for the others.
how can I do this?

configure a queue for this person and place all calls that should go to this person in it.

thank you for your answer,would you please explain more?
I use freepbx on trixbox v.2.2.5 ,Asterisk 1.2.24

ok that helps to start. But how do calls get into your system? trunk types, possible providers, country you are in?

details… If you don’t say what you are using then we have to guess? POTS lines? using who’s card? T1? E1? PRI? ISDN? SIP? IAX?

thank you for your guidence,I configured the queue and it worked.
I am from Iran,using TDM400,E1, my trunk type is the default zap/g0