why is "tu" necessary? Can't I just use "manges" since that's the second person form of the verb? That's how it is in Spanish and in Portuguese, at least...
Unlike Spanish and Portuguese, in French you have to use the pronoun. In French, when said out loud, the je, tu, il/elle/on, and ils/elles conjugations are pronounced the same, so you need the pronoun to differentiate.
"You eat a tomato and some fruits" isn't good English, at least not in the way it's used here. You could say, "You eat a tomato and some fruit," and the next thing you know, you turn orange. But that's not the sense here. Also, we would say "some fruit," not "fruits."