follows these step:- 1. write a vf page by using Standrad Controller as the object name of object which's button you want to override, 2.then go to set up page of your object then on that page go to "Buttons, Links, and Actions" 3. edit del button and choose your Override with you vf page.
http://stackoverflow.com/questions/10317437/override-a-standard-button-in-salesforce-page
http://salesforce.stackexchange.com/questions/2046/how-to-add-a-confirm-dialog-to-a-command-button
1. write a vf page by using Standrad Controller as the object name of object which's button you want to override,
2.then go to set up page of your object then on that page go to "Buttons, Links, and Actions"
3. edit del button and choose your Override with you vf page.
Thanks
Anurag Jain