Show a Messagebox if program is ran for the first time in C#

When a user runs the program for the first time I want a message box to show up.

I was thinking of something like this:

        private void Form1_Load(object sender, EventArgs e)
    {
        if(firstTime)
        {
            MessageBox.Show("Welcome");
        }

How could I get my program to display a message box when a user launches the program for the first time in c#?