In Spanish there is a phenomenon know as the personal ‘a’.  Understanding when it is used is fairly easy.  But it is also easy to forget to use it because there is no personal ‘a’ in English.  And there is no translation for the personal ‘a’ in English.  Unfortunately, if we forget to use the personal ‘a’, it will not sound right.  People will understand, but not using it when it is called for, is not good Spanish.   Basically, the personal ‘a’ is used when a specific person, persons, or even a specific animal (a pet) is the direct object of a verb.  This means the action of the verb goes to a person, persons, or a pet,  that have been specified.  When this is the case an a, the personal ‘a’, is placed before those nouns.  ‘Specific’ is the key here.  For example:

If I am looking for my secretary (who already works for me) a personal ‘a’ would be used.  She is specified.
I’m looking for my secretary. - Busco a mi secretaria.  Or, estoy buscando a mi secretaria.
If I am looking for a secretary (to start work for me), the personal ‘a’ would not be used.  She is not specified.
I am looking for a secretary. - Estoy buscando una secretaria. Or, busco una secretaria.  
It is the same idea with a pet.  If I am looking for a pet (that I own or know), a personal ‘a’ would be used.
I am looking for my pet. - Estoy buscando a mi perro.
If I am looking for a pet to rescue and own for example, the personal ‘a’ would not be used.
I am looking for a pet to rescue. - Busco un perro para rescatar.
Here are a few more examples:
I invited Jan to the movies. – Invité a Juanita al cine.
She is waiting for her brother. – Está esperando a su hermano. – Or, espera a su hermano.
I am looking for my sister. – Busco a mi hermana. Or, Estoy buscando a mi hermana.
He is watching his dog play. Está mirando a su perro jugar.
Do you see how the specific person (or pet) is where the action of the verb is going?  The action of being invited goes to Juanita, the action of being waited on goes to her brother, the action of being searched for goes to my sister, and the action of being watched goes to his dog.  All the object nouns above are known and specified.
The personal ‘a’ is also used with the interrogative pronouns, who- quíen, and whom- quíenes.   Now to me this seems to contradict the specified person rule above because you are not really specifying ‘who’ it is.  But quíen and quíenes do require a personal ‘a’.
Who did Sara call?  –¿A quíen llamó Sara?
Whom are you calling? – ¿A quíenes llama?
Who likes to drink wine? - ¿A quíen le gusta tomar vino?
Who did you invite? - ¿A quíen invitaste? (Informal).
(To) whom did you say that? - ¿A quíenes le dijo eso?
Also, there is another type of pronoun called a definite pronoun that requires a personal ‘a’.  Definite pronouns are pronouns such as no one, anyone, someone etc.  Here are a few examples:
I need someone who can read Spanish. – Necesito a alguien que pueda leer español.       
I did not call anyone. – No llamé a nadie.
She did not visit any of her friends on her trip. – No visitó a ninguno de sus amigos en su viaje.
It is also important to know when not to use the personal ‘a’.  After becoming familiar with using the personal ‘a’, it can be easy to insert it when it is not called for.  This sounds just as wrong a not using the ‘a’ when it is called for.  Here are a few basic rules to follow on when not to use the personal ‘a’:
As implied above, a personal ‘a’ is not used with unspecified people (or pets). Also, it is not used for places or things.  
I am looking for a Spanish translator. – Busco traductor de español.
I need a loyal secretary. – Necesito una secretaria fiel.
Where can I find a policeman? – ¿Dónde puedo encontrar un policía?
I see Susan’s car. – Veo el carro de Susan
They visited their home. – Visitaron sucasa. 
And, typically, the personal ‘a’ is not used with the verbs Ser- to know facts, Tener- to have possession, or Hay- there is, there are. 
She is Panamanian. - Ella es Panameña.
He has a very large house. – Tiene una casa grande.
There were only seven people at the party last night. – Había solo siete personas en la fiesta anoche.
The personal ‘a’ in Spanish is very common.  You will see and hear the personal ‘a’ quite often.  Knowing the basics of when the ‘a’ is used, like in the examples above, will help you fine-tune your Spanish.  A good way to practice its use is to look for it when you are reading.  Then try to understand why it is there.  
altGood luck and stay with it.  
Hope this helps,