Here is another confusing pair It's and Its
It's is a contraction for it is or it has.
If you can replace it [']s in your sentence with it is or it has and if it makes sense then your word is it's.
1. It's been good to know that. Contraction: It has.
The sentence will be: It has been good to know that.
2. It's a x-ray machine. Contraction: It is.
The sentence will be: It is a x-ray machine.
Its is a possessive pronoun meaning, more or less, of it or belonging to it.
Its is the neuter version of his and her. If you try to put the her or his into your sentence where you think its belongs and if the sentence works grammatically then your word its.
1. The cheetah is well-known for its ability to run fast. Possessive pronoun: its ability = the cheetah's ability.
And there is no word such as its'.