Use "it's" when you mean "it is" or "it has." Say the sentence or phrase with the words "it is" in place of "its" or "it's" to see if it makes sense.
For example: "It's hot in here" also can be said "It is hot in here."
Or: "It's been raining for days" also can be said "It has been raining for days."
Use "its" to show possession. This is tricky because nouns typically use an apostrophe and an "s" to show possession: "Mark's hat" or "Mary's cat." Remember, however, that pronouns, like "he" and "she," do not become possessive in this way. We use "his" or "her" for possessives.
For example: "The dog chased its tail." You could not substitute "it is" for "its," or you would have "the dog chased it is tail."
Use this simple reminder: It's "it's" if it is. If not, it's "its."
Take an online test or have someone write a few sentences using both "it's" and "its," but leaving those words out so that you must fill them in. See Resources.