In vb6.0 how can I display selected checkboxes into textbox..?

As suggested by @Richard-MacCutchan, build an array, sort it ("ascending order" is in your requirements), join them into a single string and add it to the text box. Here is a working example in VBA. It's very similar to VB6 and I don't have VB6 any more - it went out of support decades ago.

Dim m As Integer
m = Me.Controls.Count
ReDim TheArray(m) As String 'Max size the array will be
Dim cBox As Object, index As Integer

index = 0
For Each cBox In Me.Controls
    Debug.Print TypeName(cBox), cBox.Value
    If TypeName(cBox) = "CheckBox" Then
        If cBox.Value = True Then
            TheArray(index) = cBox.Name
            index = index + 1
        End If
    End If
Next
ReDim Preserve TheArray(index - 1)  'Get rid of the empty spaces
'Sort the array alphabetically
SortArray TheArray

Dim s As String
s = Join(TheArray, vbCrLf)
Me.TextBox1.Text = s

Note because I have joined the string using carriage return/linefeeds the text box will need to have Multi-line property = True.

Now let's address the elephant in the room...

Quote:

i am new in vb6.0.

If you are learning VB6 so that you can maintain (or rewrite) a legacy system then fair enough. But if you are learning VB6 for any other reason then please stop. VB6 is ancient technology. You would be far better off learning one of the .NET languages - and they are free to download. The transition from VB6 to VB.NET is not hard, but is not advised as you will miss out on the new way of doing things, and probably have to "unlearn" a lot of bad habits (I did it that way and have regretted it many times since)

Edit: Here is an example of sorting an array adapted from - FreeVBCode code snippet: Simplified Array Sorting[^]

Public Sub SortArray(ByRef TheArray As Variant)
    Dim Sorted As Boolean
    Sorted = False
    Do While Not Sorted
        Sorted = True
        Dim X As Long, Temp As Variant
        For X = 0 To UBound(TheArray) - 1
            If TheArray(X) > TheArray(X + 1) Then
                Temp = TheArray(X + 1)
                TheArray(X + 1) = TheArray(X)
                TheArray(X) = Temp
                Sorted = False
            End If
        Next X
    Loop
End Sub

Edit: The full set of code that will produce the results as described

Sub chkcheck()
    Dim m As Integer
    m = Me.Controls.Count
    ReDim TheArray(m) As String 'Max size the array will be
    Dim cBox As Object, index As Integer
    
    index = 0
    For Each cBox In Me.Controls
        Debug.Print TypeName(cBox), cBox.Value
        If TypeName(cBox) = "CheckBox" Then
            If cBox.Value = True Then
                TheArray(index) = cBox.Caption
                index = index + 1
            End If
        End If
    Next
    ReDim Preserve TheArray(index - 1)  'Get rid of the empty spaces
    SortArray TheArray
    
    Dim s As String
    s = Join(TheArray, ",")
    Me.TextBox1.Text = s
End Sub
Public Sub SortArray(ByRef TheArray As Variant)
    Dim Sorted As Boolean
    Sorted = False
    Do While Not Sorted
        Sorted = True
        Dim X As Long, Temp As Variant
        For X = 0 To UBound(TheArray) - 1
            If TheArray(X) > TheArray(X + 1) Then
                Temp = TheArray(X + 1)
                TheArray(X + 1) = TheArray(X)
                TheArray(X) = Temp
                Sorted = False
            End If
        Next X
    Loop
End Sub

index = index + cBox

index = index + 1

s = Join(TheArray, ",")

ReDim Preserve TheArray(index - 1)  'Get rid of the empty spaces
SortArray TheArray