Learn Italian: Sei tu.

In English, we use the word 'it' and the verb 'is'. In Italian, the verb essere (to be) always matches the person you are identifying. Since you are identifying tu (you), the verb must be sei (you are).

Why is the word order sei tu instead of tu sei?

Tu sei normally means 'You are...' and is used to start a sentence, like saying someone is tall or a student. When you want to say 'It is you' to identify someone, you put tu after the verb. This puts a strong spotlight on the person you are talking about.

The lesson says we can usually drop pronouns like tu. Why is it included here?

You are right that Italian usually drops the subject pronoun. However, you must keep the pronoun when it is the main piece of information you want to communicate. If you dropped tu here, you would just be left with Sei ('You are'), which does not make sense by itself when trying to say 'It is you.'

How would I ask 'Is it you?' using these same words?

You do not need to rearrange or add any words! In Italian, you can turn a statement into a question just by raising your voice at the end of the sentence. So, Sei tu? perfectly translates to 'Is it you?'