The only comma-rule I'm sure about is an apposition (Emma, who is a beautiful girl, loves John). Appart from that it's only guessing but I actually need that knowledge. I hoped some cool Freeper could tell me on ten lines where to use it and where not. Otherwise I'll just buy a grammar.
My question about the apostrophe was if I use it in the genitive (floridarolf's Coke or floridarolfs Coke).
Genitive: use the apostrophe between the noun and the s, unless the noun is plural, in which case you use use the apostrophe after the s. For nouns where the plural doesn't end in s, i.e. children, you put the apostrophe between the plural noun and the s. Note: People get confused about the word "its" because the genitive form (meaning "belonging to it") has an s but no apostrophe. If you see "it's" with the apostrophe, that form is a contraction meaning "it is."
As for appositives, your example really isn't one -- since it starts with "who," it's a relative clause. An example of an appositive would be: "Susie, a beautiful girl, came to my party." In both your example and mine, though, you would set the clause/appostive apart by commas because you don't need the information enclosed in the commas for the sentence to retain its meaning.