Articles can be definite or indefinite, it depends if they refer to a known object or not and they can help you identify the gender (masculine/feminine) and number (singular/plural) of the nouns they refer to, as they change depending on that.
Use articles il (singular) and i (plural) for masculine nouns that start with a consonant.
Examples: il libro / i libri (the book / the books)
il cane / i cani (the dog / the dogs)
Words that begin with z and with the groups gn, pn, ps, s + consonant need the articles lo (singular) and gli (plural)
lo zio / gli zii (the uncle / the uncles)
lo gnomo / gli gnomi (the dwarf / the dwarfs)
lo psicologo / gli psicologi (the psychologist / the psychologists)
lo specchio, lo studente, gli spaghetti
For feminine nouns, use la (singular) and le (plural) for all the nouns (without distinction with regard to the first letters.)
la mamma / le mamme (the mother / the mothers)
la casa / le case (the house / the houses)
When singular nouns start with a vowel, the article is elided and written with an apostrophe: lo/la → l’
masculine nouns: l’ elefante / gli elefanti (the elephant / the elephants)
feminine nouns: l’ amica / le amiche (the friend / the friends)
The indefinite article is only used with singular nouns and so only changes according to the gender.
The article un is used with masculine nouns starting with a vowel or a consonant
un amico (a friend)
un libro (a book)
However, with words starting with z and the groups gn, pn, ps, sc you have to use the article uno.
uno zaino (a backpack)
uno scolaro (a student)
The article una is used for feminine singular nouns starting with a consonant.
una mamma (a mother)
una casa (a house)
The indefinite feminine form also needs to be elided when followed by a word starting with a vowel: una → un’
un’ amica (an friend)
un’ automobile (a car)