The following contains MAJOR spoilers for both The Dark Tower film, and the book series it is based on. If you're not familiar with either, go do your homework and come back here when you're ready.
Stephen King's The Dark Tower series spans seven books and several decades of writing. While the first movie is intended to be the beginning of a cinematic franchise, it makes an interesting choice in including aspects of several different books in the series to create its first story. In doing so, it changes a lot. Like, an awful lot.
Not only do characters and locations appear much earlier in the film than they did in the books, but the way those characters and locations are portrayed are drastically different. It's almost impossible to mark every change in The Dark Tower so here are the major ones that really impact how the plot of the Dark Tower is truly different, and how it may play out differently as the franchise develops, assuming, of course, that it does.
Jake Chambers' parents actually like him. His mother, especially, is really worried that her son might be truly crazy. It's touching, and nothing like how Jake's mother would react in the books. Instead of a middle- or lower-class kid, as Jake is portrayed in the movie, Jake Chambers in the books is the son of a TV executive and he's fairly well off. Also, his parents have little interest in the fact that they even have a kid. They mostly ignore him, which is why Jake ultimately has no problem (much later in the books) making the decision to leave home. The Elmer Chambers of the movie died heroically. The Elmer Chambers of the book might not remember his own son's name and snorts a lot of cocaine. There is no step-father in the books.
The Breakers are children, rather than adults. The place we see at the opening of The Dark Tower is never named on screen, but in the books it's called Algul Siento. In the film, it's inhabited by children who have psychic abilities referred to as Shine. This is an amalgamation of a couple different concepts from the books. When Algul Siento is discovered, its inhabitants, who are psychically gifted, are mostly adults. However, children, specifically twins, are taken captive because the connection that twins have can be mined and used to help give the adult psychics more strength. Finally, all of this doesn't happen anywhere near this point of the story.
Doorways between worlds are technological. Jake Chambers makes his way to Mid-World via a portal he discovers inside a house in Dutch Hill. This location, and the monster inside it, are part of the The Dark Tower's third book, The Waste Lands, but in that case, the passageway between worlds isn't a keypad and a portal, it's a door. Just a door. Doors between the Gunslinger's world and our modern world play a huge part of The Dark Tower and they're found in numerous places, however, in every case, they're just average doors that just so happen to open on an alternate reality.
The Breakers are attacking the Tower directly. In the movie we see the power of the psychic children coalesce into a beam of energy that strikes the Dark Tower, causing quakes that reverberate through the worlds. The beamquakes do happen in the books, but the psychics never get as far as attacking the Tower itself in the book because they have to take down the Tower's defenses first. At one point in the film, you see Jake draw a map in the dirt: a circle, with six lines cutting through it at equal distances, the Tower standing at the center. In the novels, those six lines represent six beams that hold the Tower up and protect it. Before the Tower can be taken down, the beams must be destroyed. Either the beams aren't part of this story (technically a sequel), or they've already been destroyed.
Roland's goal is not the Tower. The Gunslinger we meet in the film has forsaken his duty to protect the tower, in order to dedicate himself to seeking revenge for the death of his father. The Roland of the novels would tell the movie version that he has forgotten his father's face. The Roland Deschain that we meet crossing the Mohaine Desert in The Gunslinger is a man of a single mind. Reach the Tower. There is nothing more important than that. We don't even know why he seeks the Tower initially, only that he does, and that he'll do whatever it takes, no matter how treacherous, in order to reach his goal. This is the biggest change made from book to movie.
Steven Deschain lives much longer. This Steven Deschain (played, in a flasback, by Dennis Haysbert) is killed by The Man in Black in front of his son, who has reached adulthood. Steven in the books rarely appears, and not at all in the first book. While Steven Deschain is killed in the events that take place prior to the first novel, it's not The Man in Black who kills him, and it happens much earlier in Roland's life. The exact circumstances of Steven's death are never addressed at all in the novels, though. You'll have to check out the Marvel comics adaptation to get that information.
Jake is from Keystone Earth. Keystone Earth is an important location in the books, but not until much later in the story. Also, Keystone Earth is not the world that Jake Chambers is from. Jake, and other characters the film has yet to introduce, are from a world that looks a lot like ours, but it turns out, it's not. Keystone Earth is a special place, which for all intents and purposes, is written to be the actual, real world, the one in which you are currently sitting and reading this story. We are Keystone Earth. Jake Chambers is a character in a book, or in this case a movie, so he's not from here.
Jake Chambers makes a very different journey to Mid-World. The version that we see in the movie is most closely related to the second time it happens in the novels. Jake's actual first trip to Mid-World happens when Jake takes a more direct route. He dies. In the first book, Roland and Jake meet when Roland comes upon a Way Station in the middle of the desert where Jake is hiding out. He has no real memory of how he came to be there and it's not until Roland hypnotizes the boy that he remembers being pushed off the sidewalk and falling beneath the wheels of an oncoming car. He woke up in the Way Station. Jake never dies in the new movie.
Roland knows who he's chasing. Roland informs Jake when they first meet that the Man in Black is actually a man named Walter. In The Gunslinger, Roland refers to the man he's chasing as The Man in Black for a very simple reason -- he doesn't know the man's name. He actually has no idea who he's chasing. He knows that he must catch The Man in Black, that he has information Roland needs in order to reach the Dark Tower, but it's not until the end, when the two finally stand face to face, that Roland learns who the man is.
Jake survives the story. With Jake's family dead in the film, he makes the decision to accompany Roland back to Mid-World. It's a much happier ending for Jake then what really happens to him in the first novel. After a treacherous trip through a mountain cave, Jake and Roland finally reach daylight on the other side. The Man in Black is waiting but as they race to catch him Jake nearly falls. With Jake hanging on for dear life Roland must decide to either save the boy and lose his chance to catch the man he's been chasing, or let the boy fall, and continue his quest for the tower. Roland chooses the Tower.
The Tower is saved. The most interesting thing about The Dark Tower film is that, at the end of it, the Tower is safe. By the end of the first Dark Tower book, we don't even know that the Tower is in danger. Yet, here, the forces attempting to take it down appear to be dealt with quite decisively. It really makes one wonder exactly where the story is looking to go from here. While this Roland may not be interested in protecting the Tower, he's apparently done it, so what now?