Questions on Ionic bonding and chemical formulas etc?

could somebody explain clearly how to make chemical formluas/ ionic formulas (whats the difference?)? i dont understand what oxidation number to use when bonding 2 elements, whats the difference is with the number of that element and the "chare it has is, and what to do with oxygen is added to it all like sodium phosphate. please explain it as clearly as possible and thanks in advance these concepts are very confusing to me. =]