@EventHandler
 public void onPlayerJoin(PlayerJoinEvent p){
     Player a = p.getPlayer();

     a.sendMessage(ChatColor.BLUE + "Welcome message");
}

yourPlayer.hasPlayedBefore();

@EventHandler
public void onPlayerJoin(PlayeRJoinEvent p) {
    Player a = p.getPlayer();

    boolean hasPlayed = a.hasPlayedBefore();

    if (hasPlayed){
        a.sendMessage("Welcome back to the server!");
    }else {
        a.sendMessage("Welcome to our server! This is your first time playing!"
    }
}