Let's see a few examples to learn the proper use of the Spanish word falta, a false friend of the English word fault.

First of all, falta does mean "fault" in the context of sports:
 

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El árbitro no vio la falta
The umpire didn't see the fault

The word falta in Spanish is also used in legal contexts. Una falta means "an offense" (the word ofensa also exists):
 

Que una misma persona cometiera distintas faltas de hurto.

That one person committed different robbery offenses.

Caption 49, Los Reporteros - Crecen los robos en tiendas

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Also, in academic or laboral contexts, una falta means "an absence." If you don't go to school tu maestro te pone falta (your teacher marks you absent). Generally speaking una falta means "a lack" or "a shortage" and the verb faltar means "to lack," "to need" or "to be absent." Study the following examples:
 

Me hace falta un aguacate que voy a hacer una ensalada, eh.

I need one avocado and I'll make a salad, uh.

Caption 43, 75 minutos - Del campo a la mesa

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Hoy estamos protestando por la falta de agua.

Today we are protesting because of the water shortage.

Caption 49, Kikirikí - Agua - Part 2

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It's interesting the way Spanish uses the word falta in expressions of time:
 

¿Qué será? Que falta un mes para la boda, ¿eh?

What would it be? That there is a month until the wedding, huh?

Caption 27, Yago - 8 Descubrimiento

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You can also use the word falta with a pinch of sarcasm:
 

Lo único que me falta es que a los diez meses empiece a caminar...

The only thing I need now is that at ten months old she starts walking...

Caption 44, Biografía - Natalia Oreiro

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In fact, the expression lo único que me falta (or lo único que me faltaba) alone, also exists, and it's commonly used sarcastically:
 

¡Lo único que me falta!

Just what I needed!

Caption 5, Muñeca Brava - 2 Venganza

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Check out the following use of falta combined with the verb hacer and negation. It's a very common way to express that something is not needed or necessary:
 

¡No hace falta un abogado!

A lawyer is not necessary!

Caption 81, Adícora - Venezuela - Darío y el Kitesurfing

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And, of course, you can use falta + hacer without negation:
 

Eres Lo que a mi vida le hace falta si no vienes

You are what my life lacks if you don't come

Captions 6-7, Café Tacuba - Eres

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Finally, a useful tip. How do you say in Spanish "It's your fault?" Unless you are playing soccer with your friends, you shouldn't say "es tu falta." For that, Spanish uses the word culpa (guilt, blame). It may sound really extreme and weird to say "it's your guilt" in English, but es tu culpa is common in Spanish: 
 

Soy el hombre al que iban a enterrar vivo por tu culpa.

I am the man who they were going to bury alive because of you.

Caption 35, El Ausente - Acto 4

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You can use es tu culpa in the most trivial situations:
 

Por tu culpa perdimos el avión, querido.

It's your fault we missed the plane, dear.

Caption 16, Yago - 4 El secreto

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Finally, another use of the word falta is in the expression faltas de ortografía (orthographic mistakes). You can combine it with the verb tener (to have) as in el ensayo tiene muchas faltas de ortografía (the essay has many orthographic mistakes), or with the verb cometer (to commit, to make) as in tú cometes muchas faltas de ortografía (you make a lot of ortographic mistakes). Thank you for reading!