sensors on missiles can be safely skipped for your first missile ship. Design your missile first, then design the launcher and Fire Control based on that missiles stats. Magazine can be whatever you think will fit, I generally do a size 10 and size 30 one. Sensor duty can and should be pulled by another ship, especially if you want to do anti-missile work. Lastly, colliers(ships with basically all magazines) will be useful to lug around extra ammo. Try to keep the size of your first ships down, this will help.
On to combat itself, Is your active sensor on? turn it on(f6 or f8).
Is ordinance assigned to your launchers(either through f6 or f8)? assign it.
Are your launchers assigned to an FC? Assign them
At this point, if you have other similar ships in this TG click the copy assign/copy TG button to propagate this change to all subsequent ships.
Does your FC have a target? pick one(has to be in range of your missiles endurance rating, your FC's scan range, and any ECM can't degrade your FC's scan range too much, if this is the case get closer).
As before, click on Copy Target/Class in TG to propagate targeting information to all ships of this class.
Have you issued a weapons free order? through f6 for each ship, or f8 for all ships order them to open fire.
advance time(typically by 5 seconds), there should be blue lines/circles with your missile names on them headed towards the badguys.