GrabCAD does not have rendering courses. There are tutorials, but those do not go into the level of detail you'd find in a proper class or course on rendering.
The "best course" is one which focuses on the software you are using. You might learn some neat tricks by studying a different program, but that is not a good way to begin.
You need to first decide what program you'll use to render. All of them have different prices, capabilities, workflows, and limitations. Try each of them. See which ones work best for you.