[aron9forever]

ok nevermind I thought you found a simple plugin but you posted a gigantic one compared to what you need
here try this

#include <amxmodx>
#pragma semicolon 1

static const WELCOME_MESSAGE[] = "Welcome to my server!";

public plugin_init() {
  register_plugin("Simplest Welcome Message", "1.0", "aron9forever aka Carnacior");
}

public client_putinserver(id) {
  set_task(3.0, "display_message", id);
}

public display_message(id) {
  client_print(id, print_chat, WELCOME_MESSAGE);
}

Just edit the text with whatever you want. Let me know if you want colors, that changes stuff a bit