Here are some tips:

It's only with people and animals

Some verbs like tener y haber don't use it e.g. Tengo una hermana.

If the person isn't a specificed person you don't need to use it e.g. necesito jardinero (refers to any garderner)

Generally though if the object is a person or animal it needs to be used.

Veo un libro (no 'a' as a book isn't a person or animal)

Veo a una chica. 

Con a y sin a.

Usamos la a personal para OD de personas, pero cuando es peraona o personas indeterminadas podenos suprimirla.

busco a un profesor...uno concreto

busco un profesor. ...indeterminado

* I meant to say 'hay'

You don't use it with this one e.g. hay dos chicas aqui

"Yo quiero conocer hablantes de Inglés."
Without the a.