Arduino Audio Input from AUX cable
Amplify the signal so that peak-to-peak voltage is within 5V but high enough for it to be meaningful to the Arduino. You can use an op-amp based amplifier using the inverting or non-inverting configurations.
Look at the inverting and non-inverting amplifiers here: https://en.wikipedia.org/wiki/Operational_amplifier_applications#Amplifiers
Offset the voltage. This can be as simple as AC coupling the output and using a voltage divider to create the offset.