Firstly Merry Christmas everyone

I have been asked by a family member to create an application, but one thing that I have decided that I should do with this application is make the UI more professional.
Up to now I have been creating applications which when I need more information I would open up a new form, but I would like to create an application which all is within the main form.

After researching this I think the solution for this is to have the different bits in different panels and then only show the relevant panels at a particular time.
But the problem I am having is getting my head around the problem with all the code. Using the panels method all the code would be in the main form, which when you have only a small amount of code then that would be fine but I'm thinking that with a larger application would make that a little more difficult as you may have hundreds of different panels.

So while I will be using OOP, so there wouldn't be much business logic code on the form, there would still be a lot.
So I am wondering if anyone had any suggestions on how I should go about doing this, if I'm using the wrong method then I would like to know what the more appropriate method would be.

Thanks for any help in advance

Satal