If you want to target them with your fire control, you need to have them to be pinged with your active radar. Passives won't work for targeting.
To pick up someone with active radar the radar needs to be turned on (from the ship list, combat overview, or a taskforce order) and the ship has to be within its scan range. The ship also has to be larger than the radar's minimum resolution. A 100HS (5000 ton radar) will not be able to pick up missiles or fightercraft or things less than 5000 tons. However such a radar would be able to see 5000 ton ships much much farther away than a 0 resolution radar would be able to see a missile or fighter if both radar's were the same size.
Once a ship is seen by an active radar and it is in your fire control, you can target it. Keep in mind missile fire controls have a resolution as well that determines their range and the size of ship they can target.
My sensor and missile firecontrol setup for my flagship:
Size 1 Missile Launcher (100) Missile Size 1 Rate of Fire 10 <----------Anti Missile Missile Launchers (AMMs)
Size 100 Missile Launcher (10) Missile Size 100 Rate of Fire 1000 <-------------Torpedo Launchers
Size 2 Missile Launcher (100) Missile Size 2 Rate of Fire 20 <-----------Anti Ship Missile Launchers (ASMs)
Missile Fire Control FC336-R100 (2) Range 1,008.0m km Resolution 100 <---------Used to fire long range torpedoes
Missile Fire Control FC357-R14 (2) Range 149.9m km Resolution 14 <-----------Normal ASM launcher control
Missile Fire Control FC336-R1 (2) Range 10.1m km Resolution 1 <-------------AMM launcher control and final emergency launch (when you use AMMs against ships)
Active Search Sensor S1050-R1 (4) GPS 1050 Range 10.5m km Resolution 1 <---------------Anti missile radar and fighter search
Active Search Sensor S1050-R100 (2) GPS 105000 Range 1,050.0m km Resolution 100 <-----------------Long range torpedo and large ship radar
Active Search Sensor S1050-R14 (4) GPS 14700 Range 147.0m km Resolution 14 <----------------Standard ASM missile radar to detect smaller ships
Thermal Sensor TH50-300 (1) Sensitivity 300 Detect Sig Strength 1000: 300m km <---------------So if a extremely fast and hot ship flys around I can see it
EM Detection Sensor EM50-300 (1) Sensitivity 300 Detect Sig Strength 1000: 300m km <--------------So I can detect enemy active radars and shields
The resolution 1 is for missiles and fighters, 14 is for the enemies 700-1000 ton gunboats. 100 is for their 5000+ ton ships. You'll probably come up with something different, but that setup has done well for me so far, most of my ships have that even if they have to use smaller (those above are all size 50) radar arrays.