separated navigation

Hi,

atm I have 1 navigation

.
It looks like:
1
1.1
1.1.1
1.1.2
1.2
2
3
and is not very nice to see.
That's why i am going to divide the navigation categories.
on the top i would like: 1, 2, 3
if you are on the home page, there may not be a left navi; only the top nivigation list.
and if one clicks on e.g. "1", on the left i want to add a 2nd navigation which shows:
1.1
1.2
if one clicks 1.1, on the left he may read:
1.1.1
1.1.2

How is this issue named, so i can try to find a tutorial which describes how to do so.

Thanks!