No olvidar los acentos

Como todos sabemos en español,  a diferencia de otras lenguas, usamos acentos o tildes. La tilde indica que tenemos que pronunciar la sílaba acentuada con mayor fuerza. Los acentos también son importantes porque indican muchas veces una función diferente (y un significado diferente ) de la misma palabra.
Por ejemplo: sí significa "yes" , mientras que si (sin acento) significa "if."

As we all know in Spanish, unlike other languages​​, we use accents or tildes. The tilde indicates that we have to stress the syllable where the accent is placed.  The accents are also important because they often indicate a different function (and a different meaning) of the same word.
For example, sí it means "yes", while si (no accent) means "if."

¿Cómo escribir los acentos en español?
(Information taken from SpanishDict website)

In Office for Windows:

For accented vowels: Press Ctrl + ‘, then the vowel (ctrl + ' + a = á)

For Ñ: Press Ctrl + ~, then the letter n (ctrl + ~ + n = ñ)
The second way is using the ASCII code. Each character in your computer has a code made up of pressing the ALT key then a three-digit number, all of which are listed below.

á = Alt + 0225

é = Alt + 0233

í = Alt + 0237

ó = Alt + 0243

ú = Alt + 0250

ñ = Alt + 0241

ü = Alt + 0252

¡ = Alt + 0161

¿ = Alt + 0191

To type the numbers, you must use the numeric keypad on the right side of your keyboard, not the number keys on the top row.

For Mac users click here 

Other option: insert  letters as symbols: