How old is your pup, how long have you had him, and how how long have you been trying to teach him to sit? When do you try to to get him to learn something new?
Puppies learn by repition. You said that he will sit if you give him a teat, so for now, stick with the Sit command. Tell him "Spot, sit." (or whatever his name is) When he sits, give him a treat, or a piece of treat, and praise and pet him. After a while, he'll be happy to sit for only praise and no treat.
I don't know how old you pup is, but I'm guessing he's still really youg ifyou're just trying to teach him sit. Puppies have a short attention span and he may likely get bored after ten minutes and no longer want to sit, treat or no treat. Give him a break and try again in an hour or so. Good luck with your new wpup.
I think you need to keep working on Sit, and do it daily for a week. If you were to take your dog to obedience school, which is onece a week, he would be taught one command at each lesson. You then bring the dog home and practice that one command during the week.
Are you useing a choke chain and a leash when you try to get him to sit?
He's more than likely nipping at your hands because to him, it's a game, not to mention he's cutting teeth.
Practice, practice and more practice and a lot of patients too! What kind of treats are you using, a low level treat or high level treats? If your dog knows sit using a low level treat and you want him to do it with no treat try rubbing a high level treat (something with a lot of odor even just a hot dog) on your fingers and tell him to sit and maybe the smell with get him to do it and if he does and you want you could pet him and tell him good boy and allow him to lick your hand. Don't know if it will work but it is worth a try, couldn't hurt. Good luck. Also you say his attention has to be on you and I can understand that Cody is the same way I don't know if yours follows you around the house like Cody does but after we learna new command like sit and wait/stay when I am folding clothes I have him sit and wait, when it is time for dinner I have him sit and wait, if I am doing the dishes I have him sit and wait, I don't make him wait until I have the chore done but he needs to do this on command and not just when we are practicing. He knows when it is practice time and when we are in the ring at puppy class and he will do everything just right but he needs to do it at different times too. Good luck. Sorry this is long.