Thread: how to create limited query

how to create limited query

I have created 3 main categories and many sub-categories for each one. No w I want that my user will be able to work (search and post) only with one category.
I.E. I want to create in the home page a link to a page where are listed only the subcategories of one mani categories and I want to create a special posta a project for each one of the mai categories.

What I have to do?

thanks
Fausto