Learn Spanish (Latin America): Amigo, tú no necesitas el teléfono.

We use tú because the speaker is addressing a friend (amigo). Tú is the informal word for "you", used with friends, family, and children. Usted is the formal "you", used to show respect to strangers, elders, or people in authority.

Why does the verb end in an "s" (necesitas)?

In Spanish, verbs change their endings depending on who is doing the action. The informal "you" (tú) usually takes an -s at the end of regular verbs in the present tense. If we were using the formal "you" (usted), the verb would drop the "s" and become necesita.

Do we have to include the word tú in this sentence?

No, you could just say Amigo, no necesitas el teléfono. Because the verb necesitas already has the -s ending that tells us "you" are doing the action, the pronoun tú is optional. It is often included just for emphasis.

Why is no placed where it is?

In Spanish, to make a sentence negative, you simply place no directly before the verb. Unlike English, you do not need extra helper words like "do" or "does". So "you do not need" just becomes no necesitas.