Forms Data Controls :: Extracting Edited Values From GridView, To Update/insert To Database?

I'm coding my first VB web application, which allows a user to input an order number and pull up invoice and shipping data linkd to that order number.

I have the first bit down and working, where the existing data in the database is displayed in the GridViews. One of the GridViews is set up to allow data editing, and I have syntax set to make the row editable, but I'm struggling with getting the "Update" link/functionality to work. I want to assign the values in each cell to an object (ie. 'dgInvNumber' for Invoice Number), but everything I've tried (been doing a ton of Google searching for help, but no luck yet) keeps giving me an "Object reference not set
to an instance of an object" error when I click the Update link for the row of data being edited.

At this point, I don't know what syntax to use to get the edited values out of the GridView row and assigned to objects for each cell (I have a total of 10 cells that can be edited in a row).

For background info, the GridView is set up with Bound Fields. I'm using Visual Studio 2005. Alot of places I've looked talk about TextBox controls and using FindControl. Also seen some places talk about DataBoundLiteral Controls. I tried the latter with no luck (still getting error). For TextBox controls, I don't understand how to use that part, since it mentions referencing the TextBox controls declared in the EditItemTemplates of the TemplateField column fields in the GridView control. But I didn't set up anything in the EditItemTemplates (that I know of).