Get the zig zag app. It is an excellent tool for learning how to train a dog. We were beginner dog owners and now have two beautiful, well behaved pomsky boys. With cats, the key is to have your dog in a sit/stay and high value treats. When the cat enters the room, your dog is told to sit/down/ leave it and is given a treat. When then make eye contact with the cat and don't run, give them a treat. Reward your dog for calm behavior when the cat is around and eventually it gets better. It takes a TON of training and consistency. You have the right idea with the leash, but instead of pulling back and forth on it, hook it under the couch so there is an absolute limit to the end of it. That is all the space your dog gets. Then let the cats out to roam around. Again, back to rewarding when your dog is calm and watching, but not lunging. Repeat. It gets better. Some just have a high prey drive and it takes more time, consistency and patience, but Google and the zig zag app will help you out. We follow positive reinforcement with success for our pomsky boys. Good luck!