I am always very tired. Also, I'm always dizzy when I stand. If I run I can feel my heart beat in my head and it hurts. I'm guessin b12 or iron?

If you are not. testing you are guessing. The only way to know if you have low B12 is/w a lab test through your doc. If you are low on B 12 please use sublingual B 12, not the kind you swallow. You will absorb much more that way. The dizzy when I stand part of your question MAY have something to do w/low blood pressure. Your doc can tell you. Peace and good health.
Get seen. You're almost a grown woman and you owe it to yourself to get a good history, physical exam and some basic labs. You may well be anemic as you have guessed. But you owe it to yourself to be evaluated, and not to shotgun-treat yourself -- I'm sure you know this already. Best wishes.
Fatigue. You might want to get seen for this symptom set. It could be many things, including anemia, but that may not explain all your symptoms such as dizziness. If your diet is balanced it would be unusual to have B12 def. at 19, and iron deficiency as well, so seeing someone who can evaluate all the possibilities may help you recover your energy.