Your question is very broad-based. There are many, many causes of anemia: bleeding, low iron, cancer, inflammation, infection, congenital problems like Sickle cell anemia, many more than these! Your doctor should work on what it might be, & if it's complicated, send you to a hematologist. It is often fixable, but one can't treat it till you know the cause.

There are many causes of anemia, the most common being iron deficiency. To treat the anemia, you must first find and address the underlying cause.