Learn Portuguese (Portugal): Eu sou o rapaz.

In Portuguese, there are two verbs for "to be": ser and estar. We use forms of ser, like sou (I am), for permanent characteristics or your core identity. Being a boy is an identity, so sou is the correct choice here.

Why is the word for "the" just the letter o?

In Portuguese, the word for "the" changes depending on whether the noun is masculine or feminine. Because rapaz (boy) is a masculine noun, we use the masculine singular word for "the", which is o. If the noun were feminine, like rapariga (girl), you would use a.

Do I have to include the word Eu (I) in this sentence?

No, you could just say Sou o rapaz. Because the verb form sou specifically means "I am" and cannot be used for anyone else, the word Eu isn't strictly necessary. Portuguese speakers often drop the subject pronoun unless they want to add emphasis, like "I am the boy."

How do you pronounce the "z" at the end of rapaz in Portugal?

In European Portuguese, a "z" at the very end of a word is pronounced like an English "sh" sound (unless the next word starts with a vowel). So, rapaz sounds roughly like "rah-PASH".