A Galactica type battlestar is armed with at least twelve missile tubes. These are located behind the "head" of the battlestar, at the front of the vessel's midsection, with the tubes' launch doors opening on the dorsal side of the hull. As the outer doors open, the missiles are moved forward into launch position by powerful hydraulic rams.
Once the missiles are loaded into the tubes and are ready to be fired, they must be armed with a target package and a point of aim. This is typically ordered by the vessel's Commanding Officer, with the order being relayed by the Tactical Officer.
Although no on-screen evidence has been presented, it is likely that Colonial battlestars have the ability to launch missiles- either at ground targets or suitable spaceborne targets- armed with conventional, non-nuclear warheads.
With Nuclear Groundstrike Missiles loaded in the tubes and ready to launch with the appropriate target information, the final stage before actual delivery requires for the Command Authority to specifically authorise the release of such weapons. Following such authorisation, any officer aboard ship with sufficient rank to order weapons fire- the Executive Officer, for example- may order the release of the missiles.