Eliana,
Listen to Patricia! Get a 'Secretary's Guide to Punctuation and Grammar' and study it. That is what I did, when I was 18. I am now 55. I have spend my entire working life as a Secretary or Administrative Assistant to large corporations. So, I know it like the back of my hand. If I did it, so can you!
Better yet, take a class, or classes. You won't regret it.