Often, they start with some sort of accident or unexpected event. For example, in 'Alice's Adventures in Wonderland', Alice falls down a rabbit hole. In 'The Chronicles of Narnia', the children stumble upon the wardrobe. It's a common way to get the human characters into the other world suddenly.
Well, they can start in various ways. Sometimes it's through a portal, like in 'Oz' where Dorothy is taken to Oz through a tornado. Other times, it could be because of a special ability the character has. In some stories, the character might be chosen or summoned to the other world. For example, in 'Percy Jackson & the Olympians', Percy discovers his demigod status which then leads him to the world of the Greek gods. It's all about creating a situation that makes the transition to the other world believable and exciting.
Most of these stories begin with the main character being in a rather ordinary situation which then takes a strange turn. In 'A Wrinkle in Time', Meg is an ordinary girl living an ordinary life until the arrival of the strange women who start her journey across dimensions. Or like in 'His Dark Materials', Lyra's life in her own world has its own rhythm until she discovers the truth about the other worlds and the device that can take her there.
There are many such stories. For instance, in 'The Magicians' by Lev Grossman, Quentin discovers a hidden magical world. It's a darker take on the typical human traveling to another world fantasy. The world has its own set of rules and dangers. Another one is 'Percy Jackson & the Olympians' series. While not strictly about traveling to another world in the traditional sense, Percy often visits the world of the Greek gods, which is a different realm filled with mythical beings and quests. And don't forget 'The Wizard of Earthsea' where Ged travels among different islands in a world full of magic.
Often, they start with the protagonist being in an ordinary situation in our world, like a normal day at school or work. Then suddenly, something extraordinary happens that transports them to another world. For example, in 'Sword Art Online', the players log into a new VR game and find themselves trapped in the game world which is like another world.
They usually begin by setting up the normal life of the protagonist. Then, there's a sudden disruption. In 'A Wrinkle in Time', Meg is a misfit in her school and home life at the start. Then the appearance of the strange women who introduce the concept of time - traveling and other worlds disrupts her normal existence. It's a common pattern to show the ordinary before the extraordinary journey into another world.
Many of these novels start with a sense of dislocation or a sudden change in the character's situation. In 'The Wonderful Wizard of Oz', a tornado sweeps Dorothy away from her home and deposits her in Oz. In 'Outlander', Claire touches a standing stone and is flung back in time to 18th - century Scotland. So, it could be a natural disaster, a magical encounter or a simple accident that sets the stage for the transportation to another world.
Often, they start with the introduction of the main character in their ordinary world. For example, in 'The Hobbit', we first meet Bilbo Baggins in his comfortable hobbit - hole. Then, something disrupts this ordinary life. It could be a message, a strange visitor, or a mysterious event. In 'The Lord of the Rings', it's Gandalf bringing news that sets Frodo on his quest.
Another common way is through a portal. The human might be exploring an old attic or a strange place and stumble upon a portal that transports them to the mlp world and during the transition, they are turned into a pony. This allows for a lot of world - building as the author can describe the journey through the portal and the new world.
Typically, these novels begin by introducing the character in a familiar world. In 'The Wizard of Oz', Dorothy is at home on the farm. Then, a natural or supernatural event occurs. It could be a cyclone in Dorothy's case, which whisks her away to the new world. This allows the reader to relate to the character's initial state before being plunged into the unknown and magical realm of the portal fantasy.
Often, they end with the time traveler either returning to their original time and having a new perspective on life. For example, in 'The Time Machine', the protagonist comes back with a deeper understanding of the future of humanity. Sometimes, they end with the traveler changing the future for the better, like in 'Back to the Future' where Marty McFly ensures his family has a more positive future.
In end of the world novels, human nature is often shown as complex. Some characters become desperate and selfish, like in 'The Road' where people turn to cannibalism. But others show great kindness and self - sacrifice, such as the father in the same book who does everything to protect his son.
Often, graveyard horror stories start with someone entering the graveyard at night. Maybe it's a curious teenager who wants to explore the spooky place or a person who has to pass through it to get home. For example, a young man might take a shortcut through the graveyard on his way back from a party, not realizing what awaits him in the darkness.