Assign Variable To Different Data Types

Putting the final touches on a project I have and the last element doesn't work 100%. the scenario: I have a function that I wrote based on the user's input of the desired column to perform the operation. As a method of error-trapping and ease of use, I set instructions for the code to activate:

1) if the user highlights multiple cells of the desired column
2) enters the letter of the column (up to "z", not case sensitive)
3) enters the number of the column

Sub Button5_Click()
Dim ColumnUsed As Variant
' find number of columns selected
For MyColNum = 1 To Selection.Columns.Count
Cols = Cols + 1
Next MyColNum
'find number of rows selected
For MyRowNum = 1 To Selection.Rows.Count
Rows = Rows + 1
Next MyRowNum..............................