ITS vs. IT’S is tricky because the way to use ITS goes against everything we're taught about possession. Here's a quick explanation that I hope will clear up the confusion:
When something belongs to someone or something (possession), we normally use an apostrophe:
- This book belongs to Gloria
- This is Gloria's book
- I enjoy reading blog posts written by Maria
- I enjoy reading Maria's blog posts
However, when something belongs to IT (or is from IT), no apostrophe is needed:
- I love that coffee house because the lattes there are yummy
- I love that coffee house because ITS lattes are yummy
- I read Maria's blog because the grammar tips are easy to understand
- I read Maria's blog because ITS grammar tips are easy to understand
IT'S is used as contraction for IT IS or IT HAS:
- IT IS getting late; I really should go
- IT'S getting late; I really should go
- How are you? IT HAS been ages since I've seen you!
- How are you? IT'S been ages since I've seen you!
See the difference? Given that ITS is an exception to the rule of possession, it really comes down to memorization to get it right. But it's important! I do a lot of reading about publishing, and as a result I often find myself checking out author websites and Amazon pages, which include anything from bios and book descriptions to blogs and Twitter posts. While that content is entirely separate from what's contained inside the pages of an author's book(s), if I see errors such as getting its/it's wrong, it affects my opinion of that person's writing.
As I've said in this space many times, if you're going to present yourself as a professional writer, make sure ALL your writing looks professional, not just what goes inside your book. Everyone makes mistakes, and sometimes a quick proofread is all it takes to catch them.