to my knowledge, there are 3 ways to do this.
teleport player to location at 0, 0, 0 in world "world"
This is good but you will have to update it every time you need to. The next skript works much better.
command /setspawn:
permission: op
trigger:
set {spawn} to player's location
send "&aSet spawn at %{spawn}%" to player
on join:
teleport player to {spawn}
With this, you can not only set spawn as many times as you like without reloading, you can also set the pitch and yaw.
set {spawn} to location at 0, 0, 0 in world "world"
set yaw of {spawn} to 180
set pitch of {spawn} to 0
teleport player to {spawn}
This skript is like the first one except you can change the direction the player is looking.
Hope this helps, and please give me rep by liking!