Nutrition
One of the main things you need to know is how important good nutrition is for giant breeds such as Danes. Great Danes have sensitive stomachs. Like other giant breeds, they are prone to Bloat or Torsion. We've been fortunate that none of our Danes or our grandpups have gotten this malady. It's one of the reasons we breed with the German influence of the wide, stout chest.
We have fed Iam's large breed puppy food to our puppies for most of the last 20 years. Before that, we used Hill's Science Diet large breed puppy food. We tried a couple of other foods we thought were great but ended up going right back to the Iam's....just do your research and get the best nutrition for your Great Dane puppy that you can. If you don't feed a good large breed formula, it will affect them later in life. They could have bone or tissue damage develop in their later years. So keep in mind that what you feed now is going to directly affect them later.
When you change your dog's food, do so gradually by putting a little of the new food with the food you've been feeding in the bowl. You change the ratio a little every day by adding a little more of the new food and a little less of the old food until you're having mostly new food and less old food. If you don't do this, and do a cold switch, it can cause bloat or torsion. We also keep ours from having a lot of activity before eating and keep them from a lot of activity after eating. No zoomies allowed right after eating. LOL....
We also stay away from rawhide treats and we make sure they can't get into anything such as the trash or the cat food. Just be aware of their surroundings and try and keep them safe.
Sunlight
You may not think so, but sunlight is very important for those fast growing bodies. So make sure your Dane is getting a suntan.