That is a really sweet and evocative sketch of the Great Dane. It has much more feeling than a super detailed finished piece likely would. There are two things that catch my eye, as a form of critique, and I only mention them becuase you do want to improve and you have so much latent talent:
the pupils don't quite match. One (dog's right eye) appears to be looking straight forward, while his other eye appears to be looking upward.
The darks are really nice in this drawing, but you might need to add one more level of darkness in order to help pop the muzzle out from the background (the shadowed neck). even if they aren't quite as different in tonal value, by making the shadow near the collar area darker, it will help make the muzzle come straight out, off the page better.
This is a very nice piece. I would suggest NOT messing with it, but just keep my suggestions in mind and move on to the next one. I bet your mom just loved it.
be sure to get some spray fixative to spray the drawings once they are finished so they cannot smudge or smear anymore. HOwever, be aware that no erasing can happen once it is "fixed". And put your signature and date in an artful location/size.
Carry on, dear!