My dog tries to eat my chickens and fights with my cat

You should always be aware that bringing a new pet home means work. And it will be more work if you already have other pets.

You need to introduce your cat and your dog to accept each other, you can find lots of posts about that around this website.

About the chickens: It is normal for a young dog to be playful and they do not know how to behave. You will have to teach him not to "play" with the chickens. Just like you have to teach young dogs to walk on a leash or to sit. There are also posts about that around here.

This situation is not only new to you. It is new to the dog, the cat and the chickens too. Take time and patience to introduce them to one another carefully and supervised.

You need to be aware that German Shepherd dogs are working animals. They are bred to herd animals, protect farms from intruders and help police officers and soldiers with their duties. These are no lap dogs.

Additionally, your dog is young and full of energy. She's like a child running around and playing on the playground without becoming tired at all. So you have a high-energy dog in a high-energy phase of her life.

And you imprison her in a house...

It's no wonder she starts attacking animals. She's simply bored and full of energy she cannot burn off. Bored dogs find the most inconvenient ways to entertain themselves, often by barking nonstop, scratching or knawing at walls and furniture and chasing anything that moves. You need to work with her, go on long walks and play games with her to make her more calm and stable at home.

Have a look at some ideas here: Alternatives to walking a dog.

Working and playing with your dog strengthens your relationship and teaches her to obey your commands.

The next step is to put her on a leash and introducing her to your chickens. This is best done after a walk or a training session so she's a little exhausted and not too excited. If you're afraid she might kill a chicken, put a muzzle on her. Go outside with her by your side and sit down on a bench or the stairs or whatever. Just be calm and feed her treat after treat with just a few seconds in between. Eating your treats must be more attractive to her than chasing chickens. Do this for just a minute or two at first and repeat the training each day. If she's too excited at first, go back inside, calm down and repeat the training an hour later.

After a few training sessions, you can reduce the amount of treats she gets. Give her one for going outside, then let her look around and give her the next treat after a minute if she's calm. After a few more days of training, calmly walk through your yard with her on the leash. You can combine this with simple obedience training. After a few more days you should be able to release her from the leash and remove the muzzle.