In Spanish, the direct object — the object that receives the action of the verb — is preceded by an a if the direct object is a specified person.

I'm looking at the television.	Miro la televisión.
I'm looking at you.	Miro a usted.

Pets or other animals we care about are also sometimes considered persons in this sense, but not all animals are.

I'm looking at the fox on the television.	Miro el zorro en la televisión.
I'm looking at my dog.	Miro a mi perro.

Persons are only preceded by a when they are specified. Below, in the second sentence you are looking at a specific doctor, whereas in the first sentence any doctor at all is fine, you don't need a specific doctor.

I need a doctor.	Necesito médico.
I'm looking at a doctor.	Miro a un médico.

Personal pronouns are preceded by a when they are direct objects.

I'm looking for someone.	Busco a alguien.
I'm not looking at anyone.	No miro a nadie.
Whom are you looking for?	¿Buscas a quién?

When using the verbs tener and hay, the direct object is not preceded by a.

I have a girlfriend.	Tengo novia.
There is a boy in the street.	Hay un niño en la calle.

Learning this might seem complicated now, but in time your intuition will tell you when to use and when not to use a.

Breakdown of Necesito médico.