Use its before a noun to show belonging: The dog wagged its tail. It's is short for it is or it has: It's cold. Both can open a sentence, so check meaning: possession needs its.
Try it: which fits, The team kept its notes or it's notes?
Paired sentences
The cat licked its paw. Here its marks the paw as belonging to the cat. Swap in it's and it is paw means nothing, so no apostrophe goes in.
The team moved its office downtown. Possession by a thing or organisation. In the same message you could also write It's moving its office downtown. Both forms sit together fine; only the possessive slot takes its.
The baby gripped its toy. English often uses its for a baby or animal whose sex is unknown or unstated. If the surrounding text already settled on she, follow that instead.
One caution
Its only sits right before a noun. The flask and its cap works, but a bare its with nothing after it is a signal to rewrite. In a message, try the cap belongs to it or restructure the phrase.
One context where you may step away from its entirely: an animal or baby whose sex you actually know. If the vet said she, write her paw, even though its paw would be grammatical. Match the reference the surrounding text has already established.