Sounds like it has already been answered but in case there is any confusion.
To get to a ship's module, once they are targeted, the 'slow' way is to go left UI panel and then tab across to sub-targets. Then scroll up / down to the sub-system you want.
It is possible to set up a hot key to allow you to cycle through sub systems on a targeted ship if you are so inclined.
I don't do it personally as I suck at combat and there is enough for me to think about in the heat of battle - managing shields, pips, ammo, target location, etc. Nevermind trying to be clever enough to take out enemy sub systems. I fly with a DS4 so it may be that I don't have the button combinations available to me to make it easy and people on HOTAS have a few extra options available to them.
I sometimes fly with Commanders who are able to juggle that extra dimension. To them it speeds up an encounter.