You need an active sensor set to Resolution 1, which will display max detection ranges both for 50 ton ships, and for various missile sizes. After that, you need to design and research some sort of non-missile weapon, such as lasers or gauss cannon. For PD, the best choice is 10cm, as you usually will just need to hit missiles to kill them. Once you have the weapon, design and research a turret (preferably quad) that houses them. You'll need to have researched a decent amount of tracking speed tech, or your turrets won't traverse quickly enough to track missiles, and you need to have turret-mounted PD because hull-mounted weapons can only traverse as quickly as the ship. For energy weapons, you should also research some of the focus tech to increase range.
Once you have the active sensor and turret designed and researched, design a Beam Fire Control system with similar range to your chosen weapon. For PD, you'll want to use the scrolldown window to select 4x traverse speed. Once you have that, design a reactor. You'll see a power requirement in the Class Design screen in the section for any energy/kinetic weapons you've added in the format of #/#. The first is the total amount of power needed to fire that weapon once, the second is the amount of power required per 5sec interval. Your total reactor output needs to be equal to or greater than the total require power per 5sec for all energy/kinetic weapons on the ship.
When you install your PD system, you should have the following:
-Power reactors providing enough power for all energy/kinetic weapons.
-Weapon turrets.
-Beam fire control.
-Active sensors with resolution 1.
You should also consider adding ECM to your ship to reduce enemy missile accuracy.
Once your ship is build, use the combat or individual ship windows to assign turrets to the fire control, and then set them to the PD mode you want, which includes maximum engagement range. You'll need to activate your sensors to detect enemy missiles.
Alternately, for AMMs, build everything the way you would for ASMs, but make a Size 1 missile with a total warhead strength of at least 1 (the bolded number in the right column, not the user input number. This will depend on your warhead tech.) For the other 3 aspects of the design, work on maximizing the to-hit % for 10km/s speeds. You shouldn't need more than 0.1 for fuel, even at low engine efficiency techs.
As before, design an active sensor with resolution 1. Then design a missile fire control with resolution one. Note that the fire control will be much smaller for the same effective range. Make sure that the ranges for these are equal or slightly greater than the range for your AMM.
The final build for AMM defence should include:
-Size-1 launchers (substantial numbers, especially for dedicated AM ships).
-Magazine(s), with substantial capacity, as you will run through AMMs VERY fast.
-Active sensor resolution 1.
-Fire control resolution 1.
-As before, strongly consider using ECM.
Once your ship is built, again, go to either the combat or individual ships window and assign launchers to your AMM fire control. Then dictate the maximum engagement range (should be less than or equal to your AMM max range). Set the number of AMMs you want your fire control to fire at each enemy missile. Note that these can be changed at any time to, say, conserve ammunition or eliminate serious threats.