Try creating a for next loop from 0 to the column count -1 and concatenate the text for the messagebox
see
For...Next Statement (Visual Basic) | Microsoft Docs[
^]
Im not sure how much other code you have as to whether your problem is getting the data from the table to the grid or just summating the column data to a string.
To summate the data use something like
Dim ms as string = ""
for x As Integer = 0 To DataGridView1.Column.Count -1
ms += DataGridView1.SelectedRows().Column(x).ToString() & vbLine
Next
Msgbox(ms)
If the issue is getting the data to the grid there are plenty of examples on the web on how to do that.
PS. Its a long time since I used VB so you will need to check my syntax