How to connect 3.5mm Stereo to crystal radio
You would connect the ground from your radio to the ground pole of the connector - that would be the ring closest to the plastic body of the plug.  The speaker connection from your radio would go to either of the otehr rings - it doesn't really matter which.

This may not work.  The output of your radio will be very high impedance (it will act like there's a big resistor in the speaker output.)  If the amplifier input is too low impedance then it will act like a short circuit to the radio's speaker output and you won't hear anything.

Also, I hope you live close to an AM radio station.  I tried several times as a teenager to build one, but could never get them to work.  We lived in the boonies, way far away from anything.  When I moved and got closer to a transmitter, I found it much easier to get a crystal radio to work.  If it doesn't work at all, you might need a much bigger antenna.

I would suggest getting a proper crystal radio earphone and testing it that way first, then moving to an amplifier later when you know the radio works.  

Given that you are planning to try this with telephone wire instead of the magnet wire generally used, I would really, really, really suggest you try it with a crystal radio earphone first.  You've got enough variables to deal with already.  Adding the amp just adds more things to go wrong.