The IF Function is the correct formula for this, You can achieve the result by using

=IF(A1="WT",0,IF(A1="AT",1,IF(A1="AB",2,""))))

provided that A1 is where your value is, you may then proceed to change the cell value to the corresponding cell in your sheet.

You can reference columns by their letter like this:

Columns("A")

So to get the column number, just modify the above code like this:

Columns("A").Column

The above line returns an integer (1 in this case).

So if you were using the variable mycolumn to store and reference column numbers, you could set the value this way:

mycolumn = Sheets("Sheet1").Columns("A").Column

And then you could reference your variable this way:

Sheets("Sheet1").Columns(mycolumn)

or to reference a cell (A1):

Sheets("Sheet1").Cells(1,mycolumn)

or to reference a range of cells (A1:A10)you could use:

Sheets("Sheet1").Range(Cells(1,mycolumn),Cells(10,mycolumn))

I think this UDF will work for you:

Option Explicit

Function ConvertLetterToNumber(ByVal strSource As String) As String

Dim i As Integer
Dim strResult As String

For i = 1 To Len(strSource)
    Select Case Asc(Mid(strSource, i, 1))
        Case 65 To 90:
            strResult = strResult & Asc(Mid(strSource, i, 1)) - 64
        Case Else
            strResult = strResult & Mid(strSource, i, 1)
    End Select
Next
ConvertLetterToNumber = strResult

End Function

It will convert each non-numeric value to its Ascii equivalent and then reduce it down to its alphabet equivalent (A=1, B=2, etc.). Just use it as a regular formula. NOTE: It assumes that your non-numeric values are upper case.

ADDENDUM

The code as stated above will return 250,000 rows in just under 2 minutes with a 4th Gen i5 processor and 8GB RAM.