A resource is a folder in resources/ with a resource.lua manifest:

resources/hello/
  resource.lua
  server/main.lua
  client/main.lua

-- resource.lua
description "Hello, world"

server_scripts { "server/main.lua" }
client_scripts { "client/main.lua" }
files { "client/ui" }      -- folders and files for pages (optional)

-- server/main.lua
events.on("playerReady", function(player)
    player:setMoney(100)
    player:emit("hello:welcome", player.name)
end)

-- client/main.lua
events.onServer("hello:welcome", function(name)
    print("The server said hello to " .. name)
end)

Add the resource to [resources] start=basic-rp,hello and restart the server.

Rules

Each resource runs in its own Lua: global variables don't clash.
Resources talk through events: events.emit on the server, events.emit on the client.
Only data travels between server and client: nil, boolean, number, string and tables.
Check everything that comes from a client on the server.
Script errors go to the server console and to the player's m2o_client.log.

Your own resource

Rules