Using one of them depends on who talks and, specially, on which region he is from.
To Spanish and Latin American people, the most universal or frequent diminutives ends in:
Example: diminutive of “delgado” is “delgadito/delgadita”
Diminutives: we use them to point out the smallest size of something or someone, its little importance. Also, we use it to show appreciation or disdain. If you pay attention to the context, you will see the differences.
Example: diminutive of “niño” is “niñito” or “niñín”
Sentence “el niño quiere el chupete” changes into “el niñín quiere el chupete” or “el niñito quiere el chupete” when we use diminutives. First sentence can mean afecction, but not in the second sentence.
In the area of Castille, the diminutive for “libro” is “librito”, but in Navarra is “librico”.
In both cases, word “libro” doesn’t loose the ending letter “-o”. It adds “-ito” and “-ico”.
You will learn some vocabulary meanwhile you create diminutives with substantives and adjectives. You can start with “-ito/-ita” and “-in/-ina” endings to become familiar with them.
Watch these examples and observe how we create diminutives with words which ends in two vowels or consonants:
camión/camioncito (it adds “cito”)
corral/corralito (it adds “ito”)