How do I "Close" one form and show another ?

Question

I have two forms.
ie. formLogin, formMain

When a user clicks a button on formLogin I want the form to close and formMain to show.(or formMain to show and formLogin to Close)

I can't find any simple help on the internet and also I want to "Close" the form rather than Hide it.

I am new to c# so please help me.

THank You