it's

When we are writing it is very easy to get confused by 'it's' or 'its'. Here is an explanation that may help avoid confusion:

It's

'It's' is short for 'it is' or 'it has' and this is the rule. If you can't expand 'it's' to 'it is' or 'it has' then you're using 'its' when you shouldn't and that is wrong.
It's been raining all week and now it's starting to snow. (it has – it is)
It's been a very difficult year for me. (It has)