Modify dynamic_sidebar for categories/subcategories

Newbie here, I am using the dynamic_sidebar. I want only the parent cateogories to show on a given page and when you navigate to one of the categories, on that page, the subcategories open up.

For example, say my homepage has this breakdown on the sidebar:

SODA
JUICE
MILK

Assume they are all categories. If I choose soda I want to see this on the sidebar in the new page:

SODA
– Coke
– Pepsi
– Dr. Pepper
JUICE
MILK

What do I need to do to do this?

Thanks,

Chris