There are up to 8 ADC channels, assigned to different pins and you have to select the appropriate one before doing the conversion. You can either select the appropriate channel and read in an array of values from that channel, or step through the channels and convert one value from each channel.
For the fastest sampling you need to do direct port manipulation. The ADC section of the processor data sheet explains all of this in great detail, but unfortunately it is difficult to read. If you wish to start by modifying someone else's code, like the OpenMusicLabs example, you will still need to consult the data sheet in order to understand all the steps involved.