In the first book, Harry Potter is a 11 yr old boy who has been living with his mother's sister Petunia and her family(Uncle Vernon and cousin Dudley) where he's told that his parent's died in a car-crash. Later on, when he gets his letter to Hogwarts, he's told by Hagrid(Half-giant Gamekeeper at Hogwarts) about Voldemort(Or You Know Who) and that he killed Harry's parents and could not kill Harry and was destroyed himself and so Harry is supposed to be a hero in the wizarding world. Hagrid takes Harry to Diagon Alley (magic market place) where there is Gringotts(bank run by goblins) , Ollivanders(place to get wands), Madam Malkins(for dress robes, cloaks etc), Flourish and Blotts(for books) , The Leaky Cauldron ( wizarding pub and entry to Diagon Alley) etc. He goes to Hogwarts by the Hogwarts Express from platform 9 3/4 where he meets Ronald (Ron) Weasley, Hermione Granger and Neville Longbottom. Albus Dumbledore is the Headmaster of Hogwarts, Minerva Mc Gonagall- Vice and Transfiguration Professor, Severus Snape- Potions Master, Death Eater and spy for Dumbledore, Flitwick- Charms, Trelawney - Divination, Filch (&Mrs Norris)- Caretaker.
In the 1st one, Harry,Ron and Hermione stop Quirrell ( who was sharing his body with Voldemort) from getting the Philosopher's stone which would have given Voldemort the Elixir of Life.
In the 2nd one, Voldemort through the 15 yr old memory of himself(Tom Marvolo Riddle) preserved in a diary uses Ginny (Ron's little sister) to get rid of the muggle-borns in the school by opening the chamber of secrets and releasing the Basilisk which kills all those who look directly into its eyes. Harry gets rid of it using Godric Gryffindor's sword and with the help of Fawkes, Dumbledore's Pheonix.
In the 3rd, Lupin comes as the DADA professor and Ron's rat is actually Peter Pettigrew (Animagus) who betrayed Harry's parents and was responsible for their death but Sirius Black(Harry's Godfather) was blamed for it and goes to Azkaban for 13 years (for also blowing up a street full of muggles along with Peter...everyone thinks Peter is dead when he is the one actually responsible for the incident and has gone into hiding as a rat until Voldemort's return). Harry discovers the truth and with the help of Hermione, a Time-Turner and Buckbeak (Hagrid's Hippogriff) helps Sirius escape as they are unable to prove him innocent because Peter escapes. Also in this year, Harry learns to conjure the Patronus where his takes the form of a stag to be used against dementors while thinking of a happy memory.
In the 4th one, The Quidditch World Cup and the Tri-Wizard Tournament takes place where Harry is made the 4th champion along with Cedric Diggory(Hufflepuff)-Hogwarts, Fleur Delacour-Beauxbatons and Victor Krum( plays Quidditch for Bulgaria) -Durmstrang. Alastor (Mad-Eye ) Moody is the DADA teacher this year who was an ex-auror. At the end of the 3rd task of the Tournament, Harry and Cedric on touching the Cup which turns out to be a Portkey reach a graveyard where Peter ends up killing Cedric and Voldemort is revived using Harry's blood thus ending the blood-protection his mother's blood provided. On returning to Hogwarts, using the same cup along with Cedric's body, he discovers that Barty Crouch Jr , Voldemort's follower had taken Mad-Eye's place and had locked up the real one in a trunk.