(I am a computer idiot.) Can you help me figure out how to see movies from my computer on my TV?

I tried to search, but couldn’t find exactly what I need.

I have an old TV. The kind with the yellow, red and white cables. I have a digital cable box (a Motorola box from Time Warner cable) that has a USB port. I have a MacBook and iTunes (obviously).

What’s the cheapest and easiest way to make this happen?