Learn Italian: Sei una ragazza.

In Italian, you do not always need to say the subject pronoun like tu for 'you'. The verb sei specifically means 'you are', so the word tu is dropped unless you want to add special emphasis.

Is it wrong to say Tu sei una ragazza?

Not at all! That is perfectly correct. However, Italians usually drop the tu because the verb sei already makes it clear who is being talked about. You would only add tu if you wanted to emphasize the person, as if to say 'YOU are a girl.'

Why does this sentence use una instead of un?

The word una is the feminine version of the word 'a'. Because ragazza (girl) is a feminine noun, you must use the feminine article una to match it.

How can I tell that ragazza is a feminine noun?

In Italian, singular nouns that end in -a are almost always feminine, while nouns ending in -o are usually masculine. Since ragazza ends in -a, it follows this very common rule.