Usually if you run your finger along the back side you will feel the nail suddenly becoming thicker. Clip 1/4 of the inch before that spot, and it should be safe.

Warning: Some dogs will jerk away at the slightest pressure. During such times it's easy to "miss" the intended target area. So you will have to anticipate this.

Since you have not clipped your dog's nails before you need to establish it as a "good experience" for them. For example I stop after 1 or 2 nails and reward with a treat. They will soon associated nail triming not only with discomfort, but also with a treat reward. If the dog is really skittish, then lead up to the nail session by pressing with your fingers on the nails. Get them use to having pressure being applied to the nail. That will also help make it less of an unknown to them.