The link that afaf12 gave has some good info. There are several ways to go about the introduction, but I would suggest the following steps:
1. Vaccinations: Before any interaction can occur, you must take the kitten to the vet and get all the proper kitten vaccines. Also, when the kitten is old enough, you need to get him/her spayed or neutered as soon as possible.
2. Fix a Room for the Kitten: If you decide to do this, the kitten needs to stay inside from now on. Inside cats on average live to be 16 years old. They can even live into their 20s now. Outside cats, however, have an average lifespan of 4 years. Fix a room for the kitten with food, water, litter box, blankets to sleep on, lots of toys, something to scratch on, and a tall cat tree (tall enough that the cat can be on the top level and your dog cannot reach the kitten). This room becomes the kitten's safety spot, where the kitten can run back to and feel safe. When the dog is in the house, the kitten stays in the room with the door closed. Of course, you go in often to pet and comfort the kitty. However, do not let the dog in at this time. Your dog can smell the cat under the door.
3. Kitten Explores House without Dog: While your dog is outside, let the kitten explore your house. This will help the kitten feel safe when the time comes to meet the dog. Have several sessions where the kitten gets to explore the house, so he/she can smell out the whole place. Be sure to put the kitten back in the room before the dog comes in the house again.
4. Treat Time: Start feeding your dog treats on the outside of the door to the kitten's room. This will help him know that, when he smells the cat, he gets something good. Do this about 2 times a day until your dog stops barking when he smells the cat or sees the kitty's paws come out from under the door.
5. Scent Transfer: Now, take one of the blankets the kitten has been sleeping on, and put it somewhere where the dog will smell it often. This will help him get used to the cat's scent.
6. Controlled Meeting: When you feel your dog is comfortable with the cat's smell and is no longer barking at the kitten, it's time for a controlled meeting. Now this part may be difficult since your dog is a German Shepherd. You need to get some sort of a gate that your dog cannot jump over or knock down. Then, place the dog behind the gate. (This is all done inside the house.). Let the kitten out of the room, and let the kitten come up to the dog as he feels comfortable. I would suggest you stay behind the gate with the dog the first couple times so that you can comfort him and give him treats. We need to get it in your dog's mind that, when I see the kitten, I get something good. Try these meetings once a day.
7. Dog on a Leash: When you feel the dog and the cat are either ignoring each other or friendly to each other, it's time for the meeting to progress. Place the tall cat tree in the main room you typically spend most of your time. Put the cat on it, so the kitten knows where it is. Put the dog on a leash that you hold TIGHTLY, take the baby gate down, and let the dog and cat continue to interact in the room with the cat tree. If the dog reacts poorly, stop the meeting. This is where the cat tree becomes important. The cat can get away and feel safe.
As you can see, this process takes time and patience. However, please do not rush the process, or they may never get along. Throughout the whole process, when the dog smells or sees the kitten, let your dog know that he is your number one pet. This is very important.
I know this is a lot of information, but it is a tedious process. I wish you, your dog, and your new kitten a happy and peaceful adventure!