What command do I use in minecraft for setting people spawn when they spawn on a map I create? Because I am host [duplicate]

All the answers you need can be found on the Wiki

The /setworldspawn <x> <y> <z> command sets the spawn for any new players who join said world, or if they do not have a spawnpoint already.

The /spawnpoint [user] <x> <y> <z> command sets the spawn point for a specific player.

If you'd need a specific point (similar to Essentials' /spawn command) to spawn (presuming that everybody's mainly in Survival), I'd recommend making a spawn platform with a command block clock that teleports anyone on that platform to the rule room and set the /setworldspawn location to that platform.

This way, new players are captured by the /tp clock (on the platform) and are teleported straight to the rule room (a static co-ordinate).

And if you'd like to force all players' spawnpoint (only works with existing players), run a clock with /spawnpoint @a <Location>.