Hi Tuck,
Fortunately, given your age, you are probably in better shape than you think. Of course the first step is to see a dentist for an evaluation. The black areas could be calculus / tartar build up or tooth decay. If it is the former it can be removed by a dentist / hygienist, but unfortunately it wont brush off. If it is tooth decay your dentist can probably treat and restore that. As a rule of thumb, if the black area(s) are depressions or a holes in the tooth it is probably a cavity. If they are deposits or convex growths on the tooth it is probably calculus.
Your dentist will likely recommend a course of scaling and root planing (deep cleanings)as its likely you may be in the early stages of gum disease.
Although you can see a Periodontist first, given what you have described, seeing a dentist first is what I would recommend.
In the meantime, you should brush and floss your teeth and use an antibacterial mouthwash such as Listerine or Crest Pro-Health. I have been recommending the latter as a personal preference.
Hope that helps! Thanks for your question.