When the direct object of a verb is a specific person (instead of a pronoun), you have to write the prepositional article a before the person, whether you have a preposition there in English or not. Basically, use the personal a whenever a person follows directly after a verb in a sentence. The only exceptions to this are with the verb hay and the verb tener. You can also use the personal a when the noun is a domesticated animal–something that a person might consider part of the family, for example. You do not use the personal a for any other animal, for a nonspecific person, for places, etc.
Conozco a Maria. I know Maria.
Ellos miran a Juan. They look at Juan.
Ella llama a su hermano. She calls her brother.
Yo peino a mi gato. I comb my cat.
Yo conozco Paris. I know Paris.
Ella llama el doctor. She calls the doctor.
El miro el león. She looks at the lion.