I think you need to clarify your goals here. To me "assign a variable in my checkbox" sounds odd. As a VB6 coder you could use that checkbox's onchange method to change a variable as you see fit but "assign a variable" doesn't seem right.
What is this "call it to my "form2"" mean? Could this be using that variable in another form? Could it be "How do I show another form in VB6? That's unclear.
That said, you need to forget VB6. No company I know is creating new apps with that old thing. No classes should be using it either. Move forward to a current version of Visual Studio. There are the Express versions so there's no excuse to learn old bad things.