Dim Lastrow ? Dim LastColumn

I am very fimilar with Dim Lastrow,

Dim lastrow As Long
    lastrow = Cells(Rows.Count, "B").End(xlUp).Row

but how can I

dim lastcolumn ? so it would look for through row 3 from A TO IV and find which column had the last value in and the when I fill right I only want it to fill to that last column.

Any help ?