"It" is used to identify something or someone.
"Who is it?" - It's me.
"What is it?" - It's a cat.
"It" is also used to talk about the weather, about time, and about distance.
Talking about weather "What's it like outside?" It's sunny.
Talking about time "What time is it?" It's two o'clock.
Talking about distances "How far is it to Banff?" It's 900 kilometers?
"There" is used to say that something exists in a particular place.
Singular There is a party tonight.
Plural There are lots of people at the party.
Singular question Is there a party tonight?
Plural question Are there many people at the party?