My new GSD girl Chloe is now 8mths old and has never been in a house let alone walked on a lead, in the month I have owned her she has started to learn many things like toilet training, house manners and of cause how to walk on a lead. As she is still a puppy it will take time and there's no hurry, I walk her around the yard on a check chain giving her gentle checks and of cause the check chain is around the right way getting her to sit and heel using plenty of treats for about 15 mins each day, when she is confident I'll take her for walks.
Chloe is my 3rd GSD and I've done obedience training with my other two from 4mths, so I have an Idea of what to do as it is much easier to start train a 10 wk old pup than a fully grown 29kg one.
She starts obedience training next year with our local club, she'll meet many other dogs and people because she needs socialization and this is the best way. If I were you Ceowulf I'd contact your local All breeds club and they will only be too happy to help and you won't look back, good luck. .