We'll need to know the scope model too, to be able to give you some advice. If it's a VX-I 3-9x40, then medium height rings should work fine. If it's a 2-7x33 or 1-4x20 then lows should be fine. If the objective lens is over 40mm, then you may need high rings.
Sorry, yes it is a 3-9X40.