You're probably better off getting help from the subforum specifically aimed at grammar help. But since your question is easy enough, I'll answer them here.
Very, very basically (I can go more in depth, but just to provide a very basic framework). O, de, and no are particles, indicating that what goes before them is a certain part of the sentence.
o indicates an object:
tabemono o tabemashita
( Food [obj. particle] ate => I ate food)
de is generally for the place where action takes place. There are other usages though, I think.
CONBINI de tabemono o kaimashita
( convenience store [de particle] food [o particle] bought => I bought some food at the convenience store)
no marks a possessive.
Watashi no enpitsu
I [possessive] pencil => my pencil
A general book on Japanese grammar will probably help you get started on particles.