This procedure will remove any apps you may have got from the Apps market and delete calender, contacts, memo etc etc, so backup before proceeding
Copy contacts to sim card if you can and make sure battery is resonably charged, I cannot be held responsible if for any data loss.
To perfrom a factory reset;
Select
1) Settings
2) Privacy
3) Factory Data reset
The handset will go through the process of resetting everything, Good luck.