How can I add an ice maker water line?
The easiest way to achieve what you are looking for with what you already have is to:


Shut off the water at the angle stop shown in your first photo
(confirm that the valve is closed by opening the cold on you sink)
Cut the copper tubing in half (the tubing between the angle stop and
the second valve)
Install the valve tee shown in your second photo (this will reconnect the cut tube)
Test (by opening the angle valve)
Slowly tighten the compression fittings if they are leaking


Here's a video on how to install compression fittings: Youtube video.