DataGridView in Billing Style

Question

I have a restaurant billing software application.In that I use datagridview for billing table.
I used itemname, quantity ,rate,amount as datacolumns...
When i enter quantity i want to automatically fill amount column(rate and item are already filled).
Which event is used for that pupose(for better working)???
Also I want to refresh datagridview for next entry after each bill printig.
How to refresh it?? refresh means looks like a new entry editing mode-previous data should be deleted..
plz help me..