My website only has 4 main categories and severial sub cats and servieral sub sub cats etc etc.

What i would like to do is as standard, when the site first opens i would the the top cat to show all the fist subs cats.

so for example

computers
...desktop
...laptops
...Macs

Hardware
...screens
...keyboards

etc etc etc

how can this be done?

just want to show the first sub cats, not all the sub sub cats etc. These should only show when a user clicks the cat etc.