C # When you open form 2 wait 5 seconds and open form 3

In particular, I find it extremely unnecessary to use a timer for this type of operation. Use a timer when you want to repeat something within a certain time, or for cases that require a little more work. You can use Task.Delay() to make your application wait for a certain amount of time without crashing the GUI.

private async void LetTheGameStart_Load(object sender, EventArgs e)
{
    await Task.Delay(5000);

    AbrirForm2(); // Coloque aqui o código para abrir o form2
}

With Task.Delay you get the desired delay. await does not lock the application.

Using Timer

If you want to insist on Timer (this may even be better in older versions of the .NET Framework) you will need to set a Interval for this timer and a Tick event. The Interval is the time (in milliseconds) that this timer will be triggered and Tick is the event that will be executed. Here's an example:

public partial class Form1 : Form
{
    private readonly Timer timer = new Timer();

    public Form1()
    {
        InitializeComponent();
        timer.Interval = 5000; //Definir o intervalo em 5 segundos
        timer.Tick += timer_Tick; // Inscrever o evento tick
    }

    private void timer_Tick(object sender, EventArgs e)
    {
        new Form().Show(); // Aqui deve-se abrir o Form2
        timer.Stop(); // Parar o timer, porque isso só é necessário uma vez
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        timer.Start(); // Iniciar o timer quando o form for carregado
    }
}