Anyone who has used this and created or modded templates could anyone help me out in regard to creating a category menu?

I have some categories each with 2 sub categories and I would like a menu that will display all posts relevant to that category AND the sub categories but I have gogogled and read up and not sure still or found what I am after

Do not need help with css and html of course, just how you get wordpress data into it.

No one have any ideas on best how to do this?