Please excuse the usual gang of jokers here. Most English speakers don't think about the language in technical terms and only have very bad memories of trying to learn things like "genitive" and "apposition" in school.
For your questions:
Yes, apostrophes are used to indicate possession. Fred's hat. The town's park.
If the noun is plural we add not an "'s" but just the apostrophe. The teachers' lounge. The players' locker room.
There are, of course, always exceptions, mostly dealing with plural nouns that don't end in "s." Don't worry about that for now.
Commas are used, in general, to seperate items in a list or to indicate that additional clauses are being added to the meat of a sentence. Fred's ball is red. Fred's ball, which he received from his father for a Christmas present, is red. Fred's ball is red, which is his favorite color.
The comma prepares us to pause in our thought and signals us that a new idea is coming. Some people use too many, some people use too few. If you re-read a sentence and find your train of thought getting confused, you probably need a comma somewhere, to seperate thoughts.
Compare: Some people use too many commas some people use too few.
Some people use too many commas, some people use too few.
As for your question about the use of "a" or "an" with a word starting with "h," it all depends on whether the "h" is silent or not. We say "an hour" but "a harmonica."
Finally, if you are going to post this type of message again (and I see nothing wrong with trying to learn English well), identify yourself as a non-native speaker. Otherwise, people are going to assume you are just trying to be funny or that you are some type of nutjob.
SD