Genitive

In some cases the genitive is formed by adding an "'s" at the end of the word - or only an apostrophe as the word ends with an s.
It's notebly used to doing so with names of persons to mark the relation of owning, sharing...
so I read "the guerillas' alliance", "the boy's bike"...
But I think also in other cases.
I'd be very pleased to known the rules.
Or at least where to find them on the internet.
Thanks in advance for helping me.