how to add sub category on each navigation tab..

i have a website http://www.myfroject.com and i am trying to add sub-category on each navigation tab.
i.e.
under “freebie” tab i would like it to show a sub-category that my readers can click like under “freebie” tab will show kids, new freebie, magazines, etc. that they can click if that falls to what they want.
i am not sure if i am wording this correctly because i am not really good at this, i just learn from the help i am getting from this forum.
thanks a lot in advance!