I have a 2 year old German Shepherd with a sensitive tummy. She constantly going off dog food and we now want to prepare dinners for her. What would give her the right balance and a healthy diet?  

First of all you must research,research,research,find unbiased sites on the net(hard,i know) buy lots of books,join feeding websites etc etc ;)
You should join as i'm sure you have a hundred questions :D You never specified if you wanted to feed raw or homecooked.

I feed raw,it consists of about45-  50%rmb's
30% muscle meat
10% offal
10-15% fruit and veggies. This is a round about amount,some weeks they may get more or less of certain things.

But all of this is no help to you if you want to feed homecooked :( However every dog,wether homecooked or commercial fed should have raw meaty bones as a staple part of their diet.