'it'

It is a personal pronoun which we use instead of she/her or he/him when we are referring to things, rather than people, in the singular. We use personal pronouns when it is not necessary to repeat the nouns or noun phrases to which they refer.
Compare the following:

I have a friend who comes from Taiwan. He is eighteen years old. I have known him since I was twelve.

Brenda is still a close friend. I haven't seen her for three years now, but she emails me every week. I miss her.

This ring is over a hundred years old. It used to belong to my mother. I gave it to my daughter when she was twelve years old.

What's the time in London now? It's 7.15 in Argentina. ~ It's 11.15 here in London. What's the weather like? ~ It's quite good. It's sunny and it's quite warm for September.

it as an empty subject

Even when there is no question or no noun preceding it, we use it as an empty subject when we are referring to weather, time, temperature, distances or current circumstances. Study the following:

It's just awful. It's the weekend again and it's been raining since early morning.

It was 31 degrees on Crete yesterday. Don't you wish you were still there?

How far is it from London to Birmingham? ~ I'm not sure. I think it's about 70 miles.

Hi, Bob. How's it going? ~ All's well, thanks. How is it with you?

Note that they and them are the plural forms of it and that they and them can refer to both things AND people:

Where are my T-shirts? ~ They're on the top shelf. ~ I can't find them.

Have we invited Peter and Jane for Saturday? ~ No. But I spoke to them last week. They're not around at the weekend.

this/these & that/those as demonstrative pronouns

As the term demonstrative suggests, this and that are more emphatic than it and can be used to draw attention to the fact or situation. Compare the following:

Don't miss Bridget Jones's Diary. It's a film that everyone should see.

Don't miss Bridget Jones's Diary. This is a film that everyone should see

She decided to invite John to supper, but not Jane, John's partner, and it really upset Jane

She decided to invite John to supper, but not Jane, John's partner, and this really upset Jane.

When they are used as determiners with nouns, they can refer to both things AND people:

These children who attended Mallory High are well-behaved but those who came here from Brambletown Secondary are just awful.

These pre-war houses are well-built, but those that were built in the 60s are very shabby.

Generally speaking, we use this/these to refer to people and things, situations and experiences that are close to the speaker or very close in time. We use that/those to refer to people and things, situations and experiences that are more distant, either in time or physically. Compare the following:

This is lovely. Can I have some more? (The meal is in progress.)

That was lovely. Thank you so much. (The meal is over.)

Do you like these ear-rings? ~ Yes, they're quite nice. But I think those you were wearing yesterday are prettier.

On the telephone and when making introductions we use this and that. Compare the following:

Hello. Who's that? This is Tom Bruce here. Is that Alison?

I'd like you to meet my friend from Germany. This is Hannelore.