How to add running application into system tray?

First, you drag on a NotifyIcon from your toolbox on to the designer. Then set the Icon property, the Text property, the BaloonTipText property and the BaloonTipTitle property. For good measure set the BaloonTipIcon property as well.

Once that's done, add an event handler to you're Form's resized event like so:

void Form1_Resize(object sender, EventArgs e)
        {
            if (this.WindowState == FormWindowState.Minimized)
            {
                Hide();
                notifyIcon1.Visible = true;
                notifyIcon1.ShowBalloonTip(10);
            }

            else { notifyIcon1.Visible = false; }
}

This will hide the form from the taskbar, show the notification icon baloon for like 10 seconds and then just sit in the system tray.

You'll then also want to add a context menu to your notifyIcon to restore the app. You can do something like this:

ContextMenu menu = new ContextMenu();
            menu.MenuItems.Add(new MenuItem("Restore",(o,e)=>{
                this.Show();
                this.WindowState = FormWindowState.Normal;
            }));

            this.notifyIcon1.ContextMenu = menu;

Hope that helps.