Q:

When to use definite article? Like "la" and "el"

This is one of those differences between English and Spanish. Although for the most part where is definite article is not used in English, it is probably not used in Spanish, due to my lack of reading, I get unsure sometimes as to whether I should use definite article. 
Like when I say 

My sister does the homework for me. 
  Mi hermana hace la tarea por mi.

A:

Some situations in which you need to use the definite article include:

The article precedes the noun, even if there is an adjetive in between.

El coche. La raqueta. El asombroso trapecista. El milagroso elixir

But never when it precedes the names of people or places

It can precede the name of people or places when (and only when) it is used to qualify them, 

La España de la edad media (but never La España). El general López (but never El López)

Be aware that some places have the article included in the name, such as El Salvador, La Habana, etc.
This includes titles of people

El rey. El Papa. La condesa.

and days of the week and languages

El lunes. El martes. El español. El noruego.

When referring to individuals of a group.

Los niños tienen que ir al colegio. El delfín es un animal inteligente

When referring to abstract beings or entities, in a general meaning.

La paciencia es una virtud. 

This includes verbs used as subjects

El esperar tanto tiempo se hace pesado. El pasear es un hábito saludable (although this could be easily omitted)

