So it is Farid, this character " ´ " called "Acento ortográfico" or "tilde" is proper to specifics words and this is depending of pronunciation, given this is not your case my recommendation is that you reads too much and you try learn it the most words possibles, this use have a pattern as next words: Canción, Prolongación, Conjugación, Contaminación etc. I mean, all words that finish with the "ión", and this mean the volume that you must give to this word.