I want to step through the program line by line to see how the program executeto find the problem i did press F8 and nothing happened? i never used excel 2010 debugger before, please help

i want to figureout why this program keeps looping and dont run correctly, thus stepping through line for line might help yet i dont know how this debugger work?
code Public Sub CommandButton1_Click()
Do
If vbYesNo = vbYes Then
Unload UserForm1
Exit Do
end if
Cells(2, 1) = TextBox1.Text
Loop Until MsgBox("Exit and Close this form?", vbYesNo)
End Sub/ code