/k/

In Spanish there are three letters which can sound /k/: C, Q and K

We have 5 vowels and you can find the sound /k/ with all of them but the letter to use changes:

A O U work together in the same way, and E I in another way. Let´s see:

Letter C as /k/

Casa: /ˈkasa/ - House
Comida: /koˈmiða/ - Food
Cuna: /'kuna/ - Cradle

so CA, CO, CU sound /ka/ /ko/ /ku/, but what happens with E and I?¿?¿?

Cerebro: /θeˈreβro/-Brain
Cisne: /ˈθisne/ - Swan

Ce and ci don´t sound like /k/!!!!
Children in school learn this rule pronouncing this: C = /ka/ /ko/ /ku/ /θe/ /θi/. You could try!

Letter K as /k/

Well, this one is very easy. Most of words with K come from other languages and it always sounds /k/, for instance:

Karaoke: /karaˈoke/
Euskera: /eu̯sˈkera/ - Basque
Kilogramo: /kiloˈɣramo/ - Kilogramm
Koala: /koˈala/ - Koala bear
Kung fu: /kuɱˈfu/

Easy, but do not get so happy. There are few words in Spanish which have this letter, what a pitty!

Letter Q as /k/

Letter Q is always followed by U and E or I, I mean: Que or Qui.
It never works with A, O , U. Let´s see some examples:

Queso: /ˈkeso/ - Cheese
¿Qué tal?: /ke tal/ -How is it going?
Raquel: /rraˈkel/ - Rachel
Quien: /kjen/ -who
Aquí: /aˈki/ - Here
Chiquillo: /ʧiˈkiʎo/ - little boy

You will never read Qa, Qo, Qu, Qua, Quo, Quu in Spanish but you could find words like "Quorum" or "Quasi" which are Latin words.

To sum up, you can find /k/ in Spanish in these ways:

CA CO CU QUE QUI
KA KO KU KE KI