When do i put the apostrophe, after and before the S? I know if it's owned by someone, you put it before the s, but why do you put it after, like s'?
The possesive form depends on the noun you want to change into a possesive form.
If the noun does not end with s then you add 's (apstrophe s):
dog - dog's
boy - boy's
If the noun ends with s (mostly in plural forms) you add only apostrophe:
boys - boys'
teachers - teachers'
As you see sometimes both forms are possible (boy's - boys') but they have different meaning.
The form 's indicates a possesive form of a singular noun (boy's = of a boy (one person)).
While the form s' indicates possesive form of a plural noun (boys' = of boys (more than one person))