A simple Google search would have got you loads of information.

From what I can see, you can either use Application.FileSearch or simply Dir
() to list the files  in a directory. There were examples of using both to
search directories within the first few hits from the Google search.

The one you decide to use will be somewhat dependent on what you are doing.

If you are changing files in the same directory that you're listing, then
there is a chance that Dir () could get into trouble. On the other hand,
Application.FileSearch returns a collection of found files, and is less
likely to have this problem.

However, the problem can be avoided by the simple expedient of creating a
list of files first, then working through them.

In the VBA editor's help, try looking for "files collection".
then, select the entry for the
Files Property (Visual Basic for Applications)

It gives you sample code that looks like:
Sub ShowFileList(folderspec)
Dim fs, f, f1, fc, s
Set fs = CreateObject("Scripting.FileSystemObject")
Set f = fs.GetFolder(folderspec)
Set fc = f.Files
For Each f1 in fc
s = s & f1.name
s = s & vbCrLf
Next
MsgBox s
End Sub

You can modify this code to do what you need to do for each file.