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.