Armhole depth is one of the hardest measurements to get right, and it's nearly impossible to take on your own!
Here's a video from Amy on how to measure lengths, including armhole depth. Watching it will give you some guidance on the best way to measure your armhole.
If you can't watch the video, or need further instruction, here's what to do. To check your own, ask a friend or partner to run the measuring tape down your back, starting from the pointy bone at the top of your shoulder. Usually about 1" below the place where your arm meets your body is a good estimate for an armhole depth (allowing, of course, for personal fit preference).
Finally, if you're still nervous, measure the armhole on a handknit sweater that fits well. The attached photo shows how to do this.