Rules for stress in Spanish (la acentuación)
Learn the simple rules under the above link and you’ll almost never have to wonder if a vowel gets an accent.
Memorize this:
If the word ends in a vowel, “n” or “s”
The penultimate syllable gets the stress.
This means that if it ends in a consonant (excepting “n” and “s”), the last syllable gets the stress.
However…
with a few words, the stress is not on the last nor the penultimate syllable.
These words have helpful accents to show where you give the stress:
penúltimo (penultimate, next-to-last)
teléfono
número
contármelo (tell it to me)
águila (eagle)
There are others like those above, but they are not all that common.