It's really a few confuse, because you are thinking in english and trying to understand in portuguese. I got an english teacher that told me once: "to speak english, think in english" and the same advice works for you "to speak portuguese, think in portuguese". For we in Brazil, to say "eat "something" FOR breakefast" sounds weird... hahaha... it's just a cultural thing, as well as "eat "something" IN THE breakfast" sounds weird for you. There are other uses for "NO and NA" as well as described on the first answer of your question but, only with time practicing you will get familiar to the use of "NO and NA" or other words and expressions in cases like this. Some times the translation will get sense but appear really a few weird as well for we brazilians in sometimes when translating from english to portuguese.

"no" or "na" is the contraction of "em + o" and "em + a".. it's the same that "in + the" or "at".. but in portuguese, "na" and "no" will depend of the gender of your speech.. "na" for female words, "no" for male words.. eu deixei meu celular no quarto/I left my cellphone in the room.. "quarto" is male word.. minha mãe está na cozinha/ my mom is in the kitchen.. "cozinha" is a female word..