Congrats on doing the right thing. Just treat him or her with a lot of love and affection and insure for the next few months that it is not chained or tied to anything and has plenty of food and water available all of the time. Once they settle down and form a bond you can start working on getting meals into the proper time frame and work on getting him used to being on a leash. I think it is most important for a new rescue to feel secure, safe and loved. Once they understand that your their best friend they will become yours also. Try to make every interaction with them at first as positive as you can. Lots of treats and plenty of praise and you new buddy will be with you every step of the way from now on. GSD's are wary if they have been mistreated, so be patient and keep piling on the love and affection as well as lots of treats for good behavior. You can always wean off the treats later when everything has calmed down and is stable. I have two rescues who I would not trade for all their weight in gold, they are just that important to me and they know I will love and protect them for ever, so they are very relaxed and becoming great members of our family. Good luck and thanks for taking the time to do something good for a dog. You will be rewarded handsomely in the future.
Hally, The Olden Golden 12/5/1999-
Indi, (GSD) 8/4/2011-
Dixie, (GSD) 8/11/2011-
Sassy, (Rottie) Born 10/9/1999-6/12/2006 RIP