If it's showing possession, use its; if you mean it is or it has, use it's.
Test it: The dog wagged its tail (its tail, not it is tail), but It's raining (it is raining).
When the sentence is about something that belongs to it: choosing its over it's
Describing something that belongs to a thing or animal? Use its. The cat licked its paw works because you could not say it is paw there.
Writing about a city, company, or machine and showing ownership? Again its fits: The company moved its offices downtown. Nothing you could swap for it is.
Starting a sentence that states something about the weather, time, or a situation? Use it's: It's been a long week, short for It has been a long week.
Condensing it is in everyday messages? It's true that the train leaves at six reads the same with it is spelled out, so the apostrophe is correct.
Contrasting both in one sentence: The town is proud of its new library, and it's open every weekend. The first shows possession, the second shortens it is.
Here's the contrast to watch for. If you can replace the word with it is and the sentence still makes sense, you need it's, not the possessive form. It's cold becomes It is cold, so the apostrophe stays.
Its is the one possessive that never takes an apostrophe, even though every nearby form, like his, hers, or its sibling theirs, does without one in some way. When you're showing that something belongs to it, write the plain word: The phone lost its signal. The apostrophe version is always a contraction, never a possessive.
The apostrophe always signals the contraction instead. If you can swap in it is or it has and the sentence still holds, the apostrophe belongs: It's late means it is late, and It's been quiet means it has been quiet.
A quick check for your draft: read the sentence aloud with it is in place of the word. The team met its goal becomes nonsense as it is goal, so the plain possessive spelling is right there. But It's the goal that counts survives the swap, so keep the apostrophe.