The common convention seen on Django code and 3rd party apps is to
have a forms.py inside the app dir.
But It doesn't matter because Django doesn't enforce a particular file
name for this, you're free to organize your code tree as you like.
Keep in mind that Django is just Python, so if you're new to Python
you may want to understand how Python modules and imports work. Then
you will be comfortable on designing your code structure.