Here are some ways to answer this question, depending on the context:
If you need something specific:
* "Necesito..." followed by the thing you need. For example:
* "Necesito un bolígrafo." (I need a pen.)
* "Necesito un vaso de agua." (I need a glass of water.)
* "Me hace falta..." (I need...) followed by the thing you need.
If you don't need anything specific:
* "No necesito nada." (I don't need anything.)
* "Estoy bien, gracias." (I'm fine, thank you.)
If you want to be more polite:
* "¿Qué necesita usted?" (What do you need?) - This is more formal and polite.
* "Dígame, ¿qué necesita?" (Tell me, what do you need?)
To ask for clarification:
* "¿Qué necesita exactamente?" (What exactly do you need?)
* "Puede explicarme un poco más?" (Can you explain a little more?)
The best way to answer "¿Qué necesitas tú?" depends on the situation and who you are talking to.