GIVE ANSWERS - Italiano

Salve!

I've recently started learning Italian and noticed that sometimes when forming plural of some words one has to add an extra "h" (e.g. amica - amiche). I guess it's quite logical if you want to preserve pronunciation of a word, but at the same time this doesn't happen with "amico", whose plural is "amici". Is this an exception or a rule with words of different gender? Are there any other examples like in case with "amico/amica"? My Italian vocabulary scope is tiny, so I can't compare, and would be much obliged if you could clear this up for me.

Grazie!