Combo box Subform Control Default

Apologies if this is here already - I can't seem to find it and I'm seriously
stumped about how I do it.

I'm working on a main form which has a combo box and a subform control.
Depending on the option picked in the combo box, a different subform appears
in the subform control. When you open the form however, there are just empty
boxes. Is there a way to set a default subform to appear? I'm a bit of an
access newbie so all help would be very much appreciated!