If the form is bound to a table or query ... you can do this a couple of ways.
1) If the form is to be used specifically for "Data Entry" (not to view existing records), you can set the "Data Entry" property for the form to "Yes", the default is "No".
2) If the form is to be used for both viewing existing data and data entry, you can open the form in "Data Add" mode using the DoCmd.OpenForm method:
Private Sub YourButtonName_Click()
DoCmd.OpenForm "YourFormToOpenName", , , , acFormAdd
End Sub
(change "YourButtonName" and "YourFormToOpenName" to the names of your objects)
HTH
RDH