Learn Spanish (Spain): Tú eres inteligente.

The accent mark shows that tú means "you" (the subject pronoun). Without the accent mark, tu means "your" (as in "your book"). They sound exactly the same when spoken, so the accent helps tell them apart in writing.

Do I always have to include Tú when I say "You are"?

No, you can simply say Eres inteligente. In Spanish, the verb eres already tells us exactly who the subject is ("you"), so the pronoun tú is optional. It is often included just for emphasis.

Would inteligente change to inteligenta if I am talking to a woman?

No, it stays as inteligente. In Spanish, adjectives that end in -e do not change based on gender. You use inteligente whether you are talking to a man or a woman.

Why do we use eres here instead of es or soy?

The verb ser (to be) changes depending on who you are talking about. We use eres because the subject is tú (informal "you"). You would use soy for yo (I) and es for él (he) or ella (she).