Using a USB as HDD ?

Hi guys,

I've just fixed my Raspberry Pi(rateBox) and it works perfectly !
Now I am wondering how could I do to use a USB as HDD for having a larger amount of space to share my files..
The best would be to be able to use the SD card AND the USB to share files..

I am not a pro in scripting, but most of the time I understand the scripts.

Did anyone of you tried to use a USB thumb drive ?
If so, how did you managed to do it ?

Keep me in touch,
WildOnion