These are all grammatically correct english, even though they mean different things. "I see them" and "I look at them" mean almost same thing (in spanish: los veo y los miro). "I look for them" means "I search for them" (in spanish: los busco). One that would be grammatically *incorrect* is "I look them". In english we have "look at" (spanish: mirar) and "look for" (spanish: buscar). But it's wrong to use "look" without a preposition.