Curiosity is the very basis of education and if you tell me that curiosity killed the cat, I say only the cat died nobly.
Actually, it IS very easy:
you say:
It is a cat (pointing at the cat) and btw, you mean it's not a dog
but
There is a cat on the mat
(you want to say what is on the mat)
next comes the plural form:
They are cats (once again - not dogs
)
There are cats on the mats (there are many cats, at least more than one)
The construction "it's" can be be used much more widely:
It's raining/snowing
It's hot/warm
It's lovely/ugly
Almost all of them can be featured like that:
it's + a noun
it's + V (verb)ing
it's + adverb
you say:
It is a cat (pointing at the cat) and btw, you mean it's not a dog

but
There is a cat on the mat

next comes the plural form:
They are cats (once again - not dogs

There are cats on the mats (there are many cats, at least more than one)
The construction "it's" can be be used much more widely:
It's raining/snowing
It's hot/warm
It's lovely/ugly
Almost all of them can be featured like that:
it's + a noun
it's + V (verb)ing
it's + adverb