There is no such concept, "call a form". Besides, if you are new to WPF, you may not know many WPF features and techniques which could free you then need to use
in WPF at all. So, my first advice should be to develop all your applications in pure WPF, without any forms.
But if you really need to use Forms in WPF (even though your description does not justify it even a bit), you can do it. Please start here: https://msdn.microsoft.com/en-us/library/vstudio/ms751761%28v=vs.100%29.aspx
By the way, the other way around is also possible: https://msdn.microsoft.com/en-us/library/vstudio/ms742215%28v=vs.100%29.aspx
And finally, just for completeness: I have done a little research to validate: you can use the following rather exotic possibility: in a single process and single application, you can run one UI in one thread, and another UI in another thread, or you can have more threads like that. This way, one thread can run WPF UI, and another one — Forms UI. This is even easy to do. I'm not saying it's of any practical significance though.