Modifying a Input Box and Its Actions

OK, I have some code for an input box seen here:

Sub AddUnits()
On Error GoTo HandleError
Dim units
Dim newBox
newBox = InputBox("Please enter units", "Units")
HandleError:
End Sub

There are several things I want to do to it but am not quite sure how to achieve them. I have listed them below...

1. I want it so that when the Macro is activated the input starts at Cell 1 in Column C and descends down the column as data is enter. (Example: from Cell 1 to 2 to 3 to 4 and so on as data is entered.)

2. I want it so that when a number of units is entered in the message box for a cell the data in that cell is automatically added to the number entered in the input box. (Example: 25 is in Cell C5 and 10 is entered in the input box, then 35 would show up in Cell C5 and the input box would move to cell C6)

3. I want it so that when a letter is entered in the input box the input box will move to a different column and start at the first cell in that column and do the same thing as the others. (Example: The input box is in Cell C20 and A is entered, the input box would then shift to cell J1 and data entry would begin there.

4. I also have some code that will close the program and save, is there anyway to make code to put a button on the input box for that?

If you even know part of this that would greatly help. Any suggestions or assistance is greatly appreciated.

Sub macroForEddie()
Dim myCell As Range
Dim myString As String
Dim myValue As Single
    
    
    Set myCell = Range("C1")
    
    Do
        myString = InputBox("", "enter something for cell " & myCell.Address(False, False))
        If IsNumeric(myString) Then
            'user entered a number
            myValue = Val(myString)
            myCell = myCell + myValue
            myRow = myCell.Row
            'if we are out of rows, move to the next column
            If myRow > 2 ^ 16 - 1 Then
                Set myCell = Cells(1, myCell.Column + 1)
            Else
                Set myCell = myCell.Offset(1, 0)
            End If
        Else
            'user entered a letter
            Set myCell = Nothing
            'is this a valid column address?
            On Error Resume Next
            Set myCell = Range(myString & 1)
            If Err Or myCell Is Nothing Then
                Err.Clear
                GoTo leave
            End If
        End If
        
    Loop While myString <> ""

leave:
    myChoice = MsgBox("Do you want to save?", vbYesNo, "Exiting Eddie's Program")
    If myChoice = vbYes Then
        ThisWorkbook.Save
    End If
    ThisWorkbook.Close
End Sub