There are alot of experianced people here and lots of good information in the forums. Read, as questions, don't rush things.
I personally have a general rule before expecting a sub to do anything I experiance myself. Some times this means getting a third party to help out but by feeling how a whip or a toy feels it gives you a good referance as to what your sub will feel. Everyone has different pain tolerence levels and every tool feels different. Try things. Have a safe word. Talk things over before and after a play sesion.