How to read a cell value form a WPF DataGrid?

I search online and try any possible combination and nothing works:

Datagrid.Cells[..], DataGrid.Items.Cells[..], DataGrid.Rows.., DataGrid.Items.Row.. nothing works, I can't find it is MSDN or I don't understand it. I just need to read a value off a cell in DataGrid that simple.

How to do it?

thanks!