When to Interface...not to Interface?

I am trying to produce a 'Super Class'. The Top Level class will allow access to sub-classes along the lines of the Word or Excel Objet libraries.

I know that the best way to maintain this (once built) would be to implement interfaces and Binary Compatibilty, but I am unsure about how to create an interface for a Collection Class!

My Class will have a top level called CProdReqs which is an implementation of my IProdReqs.

CProdReqs exposes a CProduct class (Which needs to be held in a Collection Class of CProducts).

I know how to create regular interfaces, but not one for a COllection Class.

Thanks