public class Announcer extends JavaPlugin {

    public void onEnable() {
        Bukkit.getServer().getScheduler().scheduleSyncRepeatingTask(this, new Runnable() {
            public void run() {
                Bukkit.getServer().broadcastMessage(ChatColor.GREEN + "This is an announcement!");
            }
        }, 60//delay after command, 100//interval);
    }
}