While introducing oneself, which should one say, "I am an account manager, or "I am the account manager"?
If you are the only account manager within the context referenced, then you would say I am the account manager. If there are others beside yourself, you should say I am an account manager.
E.g. assume you are the only account manager in the company you work for. Someone is asking about the company, and she asks what your position is. Then you should say I am the account manager. Suppose that you then go to a party, and someone asks you what you do for work. In that case, you would say I am an account manager (giving your general occupation), or I am the account manager of [the company I work for] (giving the position you hold within your company).
Both are correct depending on what you are trying to communicate.
"The" communicates "one." Thus, "the account manager" communicates that the organization has only one account manager, and you're it.
"A" and "an" communicate one thing from among multiple things. Thus, "an account manager" communicates a reference to multiple account managers, of which you are one.
Similarly. You can say "I saw a bear. The bear was angry." In this example, you saw one of multiple bears. The one bear that you saw was angry.