server.properties has around sixty settings and you will only ever touch a few of them. These are the ones that change what players experience.
What to do
gamemodeanddifficultygamemodetakessurvival,creative,adventureorspectatorand applies to players joining for the first time.difficultytakespeaceful,easy,normalorhard— onpeacefulhostile mobs never spawn.max-playersA hard cap on simultaneous connections. Raising it does not give the server more memory, so raise it alongside the plan, not instead of it.
pvptrueorfalse. Turning it off stops players damaging each other but not mobs damaging players.spawn-protectionA radius in blocks around spawn where non-operators cannot build. Set it to
0if you want players to build freely at spawn.
Restart the server after saving. Changes to this file are read at boot, so editing it while the server runs does nothing until the next start.
Do not edit server.properties while the server is running. It rewrites the file on shutdown and will overwrite your changes.