Help with datagrid

Question

Well first of all sorry if this is a repeat question or question on the wrong section. Here is what i want to do: create a invoice, i am using datagridview. it has columns like. "item" "particulars" "rate" "qty" "price". user inputs the item and rate so when he makes change to the qty the price column should have value calculated as rate*qty. I also want to show the grand total on a textbox. I need help asap. PLEASE