Yelling or making this into a contest of wills is not going to get you anywhere. You are going to need to change tacks. What does she really, really like? What toy, treat, game? What is it that will motivate her? There is
ALWAYS something that a dog likes and something that
WILL motivate her. When you ask for a sit, it has to be more rewarding to her to sit than to not sit. Training needs to be fun, motivating and rewarding. What kind of treats are you using? You don't stop using treats until the dog has a skill down pat, is doing it every time easily and willingly, until it becomes habit. Then and only then, do you start on a variable reward schedule.
What I use when I'm starting to teach a skill is a hotdog. You cut it down the length in half, then down the length in half again, then crosswise into tiny, tiny, pea sized pieces. Microwave for a minute on high and soak up the extra moisture and grease in paper towels and cool. Refrigerate what you don't use for a later time. You can even freeze a bunch so they're ready. You can also use mozerella cheese or fresh meat from dinner the night before. Don't use regular, run of the mill dog biscuites or kibble. Work with your dog when she's a little bit hungry. Make it into a game, fun, exciting. Make the sessions very short. Too much repitition can bore a dog, especially an extra intelligent dog. So vary the skills and throw in some play inbetween.
Start indoors where there are not any distractions.
Ask your dog to do something she
IS good at first. Does she like to run after a toy and grab it? "Fetch." Coax her to bring it to you. Does she come if you run the other way and pat your thigh? If she comes along, "let's go," reward with a piece of hot dog. Get her excited to "work" with you.
Then try the sit. Hold the treat in your closed hand (she'll smell it, but not see it) over her head and push back...not so high that she jumps up. The second her butt hits the ground, praise lavishly and give the treat. Then toss her toy or do something else and come back to doing another sit. Don't worry about saying "sit" until you get the behavior fairly consistantly.
Keep the lessons very short (10 minutes) or as long as her attention span lasts with a young pup, but do them a few times a day. End the session on a good note...with something she is good at.
Have you looked into clicker training? It's very fun for a dog and owner. It's concise in that it tells the dog exactly what it was they just did that earned them the reward. I highly recommend it.
http://www.clickerlessons.com/index.html
Just remember that she is a baby and it's very important to let a puppy be a puppy and not take things too seriously. Training has GOT TO BE FUN and not too serious for a while...a few basic skills to get started is all she needs.
Try not to look at these puppy things as problems. She's just being a puppy and doesn't know how to work with you yet. It's a long process. She's not a problem dog. She's just being a dog. She'll get there as she matures and with patience and good training methods. Hang in there and learn all you can.