it's, its

Its or its means belonging to it--
See the dog. It is wagging its tail.
This is my book. Its cover is torn.

It's or it's is a contraction ( short, informal form ) of "it is"--
How's the weather? It's raining.
Don't eat that doughnut. It's mine.

Now that you know that, you can say--
"See the dog. It's wagging its tail."

Note--"it's" may occasionally be a contraction for "it has"--
"It's been a long, long, time..."