Presente

		singular			plural
1st person		(io) sono	I am		siamo	we are
2nd person		sei	you are (singular)		siete	you are (plural)
3rd person		è	he/she/it is		(essi/esse) sono	they are

A first important thing to remember is that Italian verbs do not necessarily need a personal pronoun, since all different persons (singular and plural) have a specific inflection. So, when the subject of a sentence is a personal pronoun, it may often be omitted. The sentence might therefore appear without a subject, because in English it is always mentioned, but in Italian the verb's inflection is often sufficient to understand who carries out the action. Only inflection sono occurs in two persons, 1st singular and 3rd plural, and might be mistaken. But also in this case, there is no need to use a pronoun when the other parts of the sentence make it clear to whom sono refers:

sono alto = I am tall (alto is singular, therefore sono can only be 1st singular person) sono giovani = they are young (giovani is plural, therefore sono is 3rd plural person)

Pronouns, though, may be used to give a certain emphasis, or to show a contrast. Focus this case:

sei un uomo = you are a man tu sei un uomo = you are a man

The second sentence might carry a meaning of you are a man, not me, or her, or you are indeed a man, not a woman, giving a certain stress to "you". Also when the sentence expresses a contrast, personal pronouns may be used for the same reason explained above:

io sono un uomo e tu sei una donna = I am a man and you are a woman noi siamo veloci, voi siete lenti = we are fast, you are slow

But if no emphasis is required, Italian often omits the pronoun:

sono magro (alternatively io sono magro) = I am thin sei un uomo (alternatively tu sei un uomo) = you are a man siete ragazzi (alternatively voi siete ragazzi) = you are boys è un vecchio libro (alternatively esso è un vecchio libro) = it's an old book è una brava insegnante (alternatively ella è una brava insegnante) = she is a good teacher sono vecchie (alternatively esse sono vecchie) = they are old (feminine)

So, there is no need to worry about the choice of pronouns. But should the latter be used for more emphasis, they need to match the verb by gender and number. Plural forms will obviously be told by the same verb inflection:

sono giovane = I am young siamo giovani = we are young sei alto = you are tall (masculine) siete alti = you are tall (plural)