Apostrophes do exactly two jobs: they show ownership, and they replace missing letters. Get that straight and you'll never lose a mark on them.
1. Ownership
"The dog's bowl" = the bowl belonging to one dog. "The dogs' bowl" = a bowl belonging to multiple dogs. The apostrophe sits where the missing word "of" would have gone.
2. Contraction
"It's" means "it is" or "it has". It never means anything to do with possession. Possessive "its" has no apostrophe — like "his" or "hers". Practice now if this still feels fuzzy.
The common trap
Plurals never take an apostrophe. "I bought three apple's" is wrong. (Unless we're talking about "Apple's CEO" — i.e. the company's CEO.)