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.
|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.
|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.