Who are the main characters introduced in Chap. 1?

The main characters introduced in Chap.1 were Polly Plummer and Digory.

How do they meet?

One morning Polly was out in the back garden when a Digory scrambled up from the garden next door and put his face over the wall.

Why has the boy been crying? 

Because his father was away in India - and he had to come and live with an Aunt and an Uncle who's mad (who would like that?) - and they were looking after his Mother - and his Mother was ill and was going to - going to - die.

What is the setting (where does the story take place, and what time of year is it)?

The story was took place in London, long long ago.

How does the weather affect the adventure? 

It was one of the wettest and coldest summers there had been for years. That drove them to do indoor adventure.

What did the girl discover in the box-room (storage room) in the attic?

Polly had discovered long ago that if you opened a certain little door in the box-room attic of her house you would find the cistern and a dark place behind it which you could get into by a little careful climbing.