how to get my dog to sit?

What exactly is not working? Have you taught the dog to sit? If not, you'll need to teach him what the command means:

1 - Stand with your dog in front of you.

2 - Have a treat or toy in your hand.

3 - Say "Sit" - firmly and in a low tone.

4 - As you say "sit", lure the dog's nose up with the treat or toy. Usually this will make the dog sit. If it walks back instead, say "No" firmly and calmly", then begin again.

5 - When your dogs sits, say good dog and reward (treat or toy)

Tip - Get in the habit of using a release command like "OK" when the dog can relax, because otherwise they might learn that "Good girl" is a release command, which it is not. I found that to be a nuisance later on, and a pain to train my dog out of!

In order for a dog to be more likely to obey a command once it understands it, it needs to see its handler as dominant. A dog that thinks it is in charge of your is likely to not obey at all, or obey inconsistently. If your dogs knows "Sit" but is just not sitting for you, just keep on at the training, be firm and it will come right. If you think your dog is becoming a handful for no particular reason, talk about it with your trainer and/or vet - there may be a reason for it.

Say sit if he doesn't take a treat and show him say sit again. If he still doesn't take the treat and hold it above his head then towards his back. He will follow the treat with his head, making him sit. Keep saying it and when he does give him the treat. It may take a few days to stick.

Take a treat and hold it over his head and slightly behind his head. He will sit so he can get a better look at the treat. When he sits, say "yes" and give him a treat.

take a treat, hold it right at his nose, then pull it back over his head. he'll follow the treat with his nose and end up sitting down. as soon as his but touches the floor say sit and give him the treat. soon enough he'll be matching sit to but-on-the-floor to treat, and will be glad to sit for u!

show him that you have a treat in your hand and point your finger from your other hand down to the ground and say in a corrective attitude SIT. then when he does reward him with his treat