Just turn on cheats, stand where you want the player to spawn, then type /spawnpoint. Just take note that if you do this while standing on a pressure plate, it will glitch up and not work, so if you're standing on a pressure plate, make sure you set the spawn a couple of blocks above it.
How would you get the players to spawn in the same spot even when they create a new world? Eg. I could make a structure and make them spawn in it and when they create a new world they spawn in that exact structure again with a completely new generated world.
Edit: I am making a modpack so thats why I want to know. I don't want them to just spawn in the same world over and over again.