What should I do if there is a huge ulcer in my teeth?

In your teeth? Ulcerations occur in soft tissue (like the gums, lips, throat etc) not on teeth. Maybe you're looking at a large cavity which would be a hole in your tooth. Whatever your problem, you should see a dentist for a thorough evaluation and treatment specific for your problem.
Seek attention. Hi, any type of ulceration in the mouth is not normal and needs to have professional evaluation. A "huge" ulcer could be a problem that could get worse. Please see your dentist as soon as you can.