Two Forms in c#

Question

Form1 will be my main form. When I click on a button, form2 appear. What can i do to make it in a way that the user can't click on form1 till the user close form2 ? Coz form1 fill the whole screen and when the user click on the button and form2 appear and if the user accidentally click on form1, form2 will just hide behind form1. This will lead to user misunderstood that form2 is missing.