Please refer to the DataGrid Class page on MSDN. From that page:
Selection
By default, the entire row is selected when a user clicks a cell in a DataGrid, and a user can select multiple rows. You can set the SelectionMode property to specify whether a user can select cells, full rows, or both. Set the SelectionUnit property to specify whether multiple rows or cells can be selected, or only single rows or cells.
You can get information about the cells that are selected from the SelectedCells property. You can get information about cells for which selection has changed in the SelectedCellsChangedEventArgs of the SelectedCellsChanged event. Call the SelectAllCells or UnselectAllCells methods to programmatically select or unselect all cells. For more information, see Default Keyboard and Mouse Behavior in the DataGrid Control.
I have added links to the relevant properties for you, but I’m out of time now, so I hope you can follow the links to get your solution.