Before wanting to wipe your device...
try this: How do I free up memory on my device? - BlackBerryFAQ
Check your device free memory by going to options > status > file free
If you want to wipe your device and are sure of it, you can do so by going to options > security options > general settings > hit menu key > wipe handheld > type blackberry.
***make sure you perform a backup before wiping because any and all information will be erased and everything in the device will be set to default (like new)***