Downward dog is a pose that we see A LOT in yoga. It can be a very challenging pose at first. I've had students complain of wrist pain while doing it, for example. The following tips may help you in downward dog. For now, don't worry about getting your heels to the ground. Make sure the foundation is right first and then work on elongating the hamstrings.
I always tell students to visualize their body as an upside down letter V. This gets them to move their tailbone to the sky, and elongate the spine and legs. From here, spread the fingers as shown. Push into the fingertips and roots of the fingers. This is where most of the weight should be and will help with wrist issues. Push the mat away from you so your armpits are long. Rotate your elbows away from you and let go of any tension in the neck.
Feet should be hip distance apart. A lot of times people take their feet too wide. Your hips are not as wide as you think they are! (A rule of thumb is to stand, and look down at your feet. With your hands find your hip bones. Your feet should be directly underneath.) Continue to push out of the armpits and elongate the spine. Without moving your feet, rotate your thighs slightly inward. Reach the tailbone to the sky. Breathe.