Many vegetarians and vegans are B12 deficient because that is where you can get B12. I think you can also get it in an algae supplement.
B12 is basically synthesized from bacteria. When people eat animal products, they are getting the bacteria that was basically in that animal's gut after they got it from soil bacteria as a source of B12 (that's a rough depiction of the actual process but that's why I'm not a biologist

). Technically a vegan could do the same with unwashed produce because of soil bacteria, but there are safer, much more reliable synthetic sources (cyanocobalamin). CVS sells a gelatin free vegetarian formula of this.
Keep in mind supplements are not FDA regulated for safety and efficacy, so be careful what you take. This is why it's
important to consult a doctor about what supplements are appropriate. However in my experience most of the doctors aware of the B12 issue will give the advice to seek out a safe, reliable source of cyanocobalamin. However, if you have a deficiency, it's REALLY IMPORTANT to get it checked out because you may need a higher dose or a more bioavailable form. If you truly feel sick or weak, it could be due to this among a variety of other things completely unrelated to your diet.
But B12 deficiencies generally take a long time to become a problem (and can be a problem whether you eat meat or not - some people have a hard time synthesizing this and the role of B12 and mood disorders is sort of an emerging thing but it is being used to treat some symptoms like that). Iron is a pretty likely culprit in teenage girls, especially with the onset of menstruation. Calcium can interfere with the absorption of iron (while Vit C can enhance it) so for those who have absorption problems things can be a little complicated. But you need to have vitamin and mineral levels checked by a doctor to be sure, and to get advice on a regimen that makes sense. Although most doctors aren't nutritionists, they can either administer and analyze the results of a blood test to check for deficiencies or refer you to someone who can (pediatricians don't always do this, in my experience).
I'm not really going to address soy as hormonal birth control except to say I wouldn't be all that concerned and I certainly wouldn't count on it. But on that note, maintaining B12 and folic acid levels is important for woman of childbearing age.