Learn Spanish (Latin America): No veo nada en la caja.

In Spanish, double negatives are perfectly normal and required! When a negative word like nada (nothing or anything) comes after the verb, you must put no before the verb. So, no veo nada translates literally to "I don't see nothing," but in English we say "I do not see anything."

Could I just say veo nada without the no?

No, you cannot. If nada comes after the verb, Spanish grammar strictly requires the no before the verb to complete the negative idea.

Where is the word for "I" in this sentence?

The word for "I" is yo, but it is left out here. The verb veo (I see) already has an ending that tells us exactly who is doing the action. Saying yo no veo nada is grammatically correct, but dropping the yo is much more common and natural.

Does the word en always mean "in"?

En is a very useful word that can mean "in," "on," or "at" depending on the context. Here, because we are talking about la caja (the box), it naturally translates to "in."