Modifying categories.

is there a way to modify the original categories & sub-categories that follow ? When u use the original ones, it basically looks like

– Random category
– One
– Two
– Three
– Etc

Basically I want the same thing but I want the user to click the categories for the sub-categories to pop down. Is there any way to do this ? Ive tried looking for it all over google for quite some time and didn’t find anything really which is why I’m asking the question here.