The English language is absolutely mucked up about this, so if you’re constantly getting apostrophes wrong, don’t feel bad. Our rule set here is silly, and we use the same character for too many things.
First, an apostrophe is never used to make something plural.
Let’s go get some baseball’s!
That’s wrong. Always. English pluralizes by adding an s or an es, in most cases, and never with an apostrophe. Even with some weird plural like hippopotami, you don’t add an apostrophe.
You use an apostrophe to indicate one of two things:
Meaning, something belongs to someone. It’s ‘s if the someone doesn’t already end in an s, or just ‘ if it does.
It’s the Smith’s house.
It’s the Jones’ house.
Jones’s would be incorrect in that second case. It’s just Jones’ because Jones already ends in an s, and there’s, like, a shortage.
An apostrophe can also stand in for one or more missing letters in a contraction, like don’t or couldn’t or it’s. However, note here that it’s is a contraction of it is, so the apostrophe is correct. You do not use an apostrophe to indicate possession by a non-entity.
The machine blew it’s gasket.
Machines are not entities like a person or a company, and so they don’t deserve apostrophes. This is correct:
The machine blew its gasket.
That’s a hugely dumb and confusing point, but I just enforce the rules, kids. But you can have fun with it:
It’s blow its gasket.
Meaning It has (correct use of apostrophe) blown its (possession by non-entity) gasket.