Its is a possessive pronoun. One major attribute of possessive pronouns is that they don't take apostrophe ('). For example: 

The dog wagged its tail. 
The dog wagged it's tail. (Wrong)

On the other hand, it's is the contracted form of any of the following:  

i. It is...; It has...; It was...

1. It's a bright and sunny day! (It is...)

2. It's been a stressful day. (It has...)

3. It's his fault. (It was...)


I suggest you also read on how to use contractions and possessive pronouns 



Thank you for reading. The blogger, Olabanji O. Odurombi, can be contacted via SMS, WhatsApp or voice call on 08037699294.