(Tu) sei carino/a! is how you would say You are cute! in Italian. If you are talking to a man, you need to make the adjective masculine (carino) whereas for a woman, you would choose the feminine carina.
The reason I’ve put tu (you) in brackets is because in Italian, subject pronouns are optional if it is already clear who the subject is. In this case, the verb sei (are) tells us the subject is the second person.
(Tu) sei molto carina stasera.
You’re looking very cute this evening.
If you want to add a clause such as I think… (Io penso…) in front of this phrase, you need to use the subjunctive sia (are). In this case, tu is necessary because sia is used for the first, second and third person in Italian and therefore doesn’t clearly specify who the subject is.
(Io) penso che tu sia carino.
I think you are cute.
Note: As in English, carino or carina are mostly used towards children or pets. If you want to give a compliment to your partner or a person you’re meeting on a date for the first time, carino may not be enough of a compliment, and can also take on a sarcastic tone if you are not careful.
The plural forms of carino and carina are carini and carine respectively.
I gatti sono molto carini quando sono piccoli.
Cats are very cute when they are small.
When referring to someone’s character rather than their physical appearance, carino can also mean sweet or kind.
Mi hai fatto un disegno? Ma come sei carino.
You drew me a picture? How sweet you are.