how come my honda xr 125cc will not start??

i go to start it and there is no power to start up, so i tured it off and checked the voltage, which showed 12v on a multimeter, but then turn the key and the volts go down but there is a click sound to the solinoid when pressing starter switch, which then makes the volts go down further, so i turn it off and the volts go back up, only using jump leads will it start....any ideas?