how to put focus on TextBox when the form load?

Set theActiveControl property of the form and you should be fine.

this.ActiveControl = yourtextboxname;

check your tab order and make sure the textbox is set to zero

You cannot set focus to a control if it has not been rendered. Form.Load() occurs before the controls are rendered.

Go to the form's events and double click the "Shown" event. In the form's shown event handler call the control.Focus() method.

    private void myForm_Shown(object sender, EventArgs e)
    {
        // Call textbox's focus method
        txtMyTextbox.Focus();
    }