Crear y escribir:
Open "C:\Miarchivo.txt" For Output As #1
Print #1, a
Close #1
Abrir y Leer un txt, lo carga en un textbox.
fnum = FreeFile
Open "C:\Text.txt" For Input As #fnum
txtFile.Text = Input$(LOF(fnum), fnum)
Close #fnum
o si lo quieres por linea:
Dim a As String
Open "C:\Text.txt" For Input As #1
Do While Not EOF(1)
Line Input #1, a
Loop
Close #1