You have done a wonderful thing in giving a German Shepherd rescue dog a second chance in life by giving it a home. Now you will need to learn how to properly care for your new dog and what things you will need to make sure that you have all of the resources necessary to have your dog cared for and your questions answered. Generally German Shepherd dogs are intelligent and loyal so you should have almost no problem getting your new pet used to its new surroundings.
The first relationship you are going to want to establish is with a reputable vet. You will be involved with a lot of people during your dog's life but nothing is more important than a relationship with a good vet. A good vet can not only care for your dog when it is not healthy but it can also offer you valuable advice on it's day to day activities and behaviors.
You may also benefit from advice from a local German Shepherd breeder when it comes to caring for your German Shepherd rescue dog. A breeder will know how to properly care for the dog and may have experience with behavioral issues that your vet may not be too familiar with. So reach out to a local breeder and explain that you may need advice on how to care for your German Shepherd rescue dog and ask if they would be willing to be a resource.
Your German Shepherd rescue dog is going to need time to adapt to its new surroundings and you are going to need time to adapt to your German Shepherd. A German Shepherd can be a dangerous dog if it feels threatened and since you may not know all of the details surrounding its ordeal you may want to give your new dog space and shower it with affection so it knows it is loved.
Housebreaking will probably be the first thing you will want to check for with your new German Shepherd rescue dog just because you really do not know where it stands on this part of training and you need to know immediately. Take it through regular housebreaking training and if it responds immediately then you will know it is already housebroken.
If it is housebroken it will take to the training with no problem. If it is not housebroken then begin regular housebreaking training and never punish the dog if it does wrong. Heap praise on it when it does right and be patient and it will understand very quickly.
If you have children you may want to keep them away, or at a distance, from your new German Shepherd rescue dog because a mistreated animal will need time to adjust and if children start poking and pulling at it then it will either sink deeper into a shell or start lashing out.
Be smart with your new dog and give it time. After a while it will be the best decision you ever made.