I have a resting heart rate of 104 i am 16 and 125 lbs. i am female and i get very dizzy when i stand. some blurred vision?

Anorexia: Something is wrong. Fast heart rate and dizzy means an illness or medical condition. I don't think just waiting it out is a smart idea.

Sinus tachycardia : I don't have your height but I am assuming your weight is within a bmi of 20-25 and you are not anorexic. My daughter is 17 and weight 122 at 5'4". Many possibilities. Excess caffeine and energy drinks, dehydration, malnutrition, cold medicine, street drugs like marihuana, hyperthyroidism, diabetes, pregnancy, dysautonomia. Talk to your parents and seek medical attention asap.

Let me also add...: In addition to the answers above, i would agree that you should get this checked out. It is not normal. Another possibility is anemia.

Just a little high: Temporary blurred vision and dizziness when standing points to orthostatic hypotension, and then a compensatory increased heart rate (tachycardia). Try avoiding dehydration and increasing your salt intake. If you have the opportunity to get your blood pressure checked while laying, sitting and standing (with the school nurse maybe?) that may be helpful.