I want to dial up a song on youtube and play along with it using my amp (and effects)through my mac laptop. I can then listen to the combo of my playing with the song (for example-Flying High Again). What cables or hardware do I need for this to work? I want to play all this through my headphones so I don't drive my wife crazy while I play. Help please?
Playing both your guitar AND a song on Youtube together won't be a problem as soon as you have a possibility to connect your guitar to your mac. If you've never done this before: The usual way to accomplish this is using an audio interface like THIS. A cheaper solution is something like THAT. The interface is connected to your mac via USB. Of course you'll also need an audio cable to connect your amp with the interface. Hope this helps – have fun!
Honestly - easiest way is to buy a USB interface for guitar. Just Google "guitar usb interface" and download Amplitube. You can run direct out into Amplitube and then use the software for the power amp/cabinet - or use the software for everything. Listen in headphones, computer speaker, monitors - whatever you have. I own a Line 6 UX1 for demo recording/practicing, and most of the time I only run my pedal board into the interface and use the software for my amplifier/cabinet/rack. A usb interface is the most important piece of gear I own for practicing, demo-ing, and just playing for fun at home.