Downward dog is a pose which we see a lot in yoga especially when flowing through sun salutations and sequences. However it can also be one of the most challenging poses at first. Beginners often find it difficult to stay in the pose however, with time and while building strength and flexibility you will eventually be able to soften into the pose.
Don't worry about getting your heels on the ground. Often I see students in a very short down dog stance because they are determined to have their heels on the ground. Try to avoid this, instead focusing on the foundation of the pose and then working on lengthening the hamstrings.
Lift your hips up and back towards the sky, this should be your highest point, keep your tailbone high. Spread the fingers wide, I often use the phrase 'wide like starfish' in my classes as it really encourages you to spread the fingers. Push into the fingertips and the tops of the hands. This will help to ground you. Make sure that your fingers are facing towards the front of the mat, this should help to protect the wrists. Let go of any tension in the head and the neck and gaze towards the back of the room.
Have your feet hip width apart and your hands shoulder distance apart. Often a lot of people have their feet too wide, draw them together so that the feet are just underneath the hip bones. Lengthen through the spine and remember to breathe!