The very cute How to Use An Apostrophe mini-site gives a quick-and-dirty “how to” for using the apostrophe. Something a large part of the English-speaking bit of the internet can use a hand with. As for ending sentences with prepositions? Well you say potato, I say tomato.
Is it plural and possessive? For Example: The soldiers’ rifles were no match for Bob’s amazing lightning pants. Then put the apostrophe after the “s”. Unless it’s a word that’s already plural, such as “children,”then you’d write “children’s.”