How to set the spawnpoint for all the players on a minecraft vanilla server?

I have a server from gratisserver.nu, so I can't use mcedit. Is there a command I can use so that all the players will spawn in the spawnpoint I have built?

Use the /setworldspawn command. This is a vanilla command. The only thing is that players won't spawn on that exact point. They will always spawn in a 30~ block radius.
Only with plugins you can make them spawn on exact points.