M.,
You need to have the heart to be firm with your daughter. She is only 12 months old, she NEEDS you to be the boss, she cannot make good decisions and that's your job.
Decide which food to feed her (I would choose a vegetable) then try to feed it to her. If she won't eat it, then take her down from the high chair. When she wants to eat again, bring out the SAME food. Don't let her have anything else but water. You don't want her to just like on milk/formula at this point.
She will scream & cry. You have have to calmly put her down from the table. Because she is young, you need to keep teaching her that the same food will keep coming until she is ready to eat it and that she won't get anything else. You probably need to remove the Cheerios from the house entirely so you are not tempted to give in.
You need to be calm and sweet even when she is being difficult. If you give in to her tantrums, you are teaching her that tantrums are how you get what you want.
It is not wise to make this a power struggle or unpleasant. Please be as calm and IN CHARGE as possible. She needs to know that you mean what you say and you don't change your mind because she cries/screams. This is a lesson that you will have to re-teach occasionally throughout her growing up. Society will thank you.
Hang in there, Mom!
D.
Mom to 8 children