Flaming Cliffs 3 is a good place to start in my opinion. Most of the interactivity is toned down a bit, and the aircraft systems are simplified.
Once you are comfortable with FC3, then choose a module with a fully interactive cockpit and train on it until you feel comfortable.
I found these videos on the F-15C's radar extremely informative and it gave me an in depth understanding of the system.
If you prefer the Su-27, then the same goes for these videos on its systems.
As for everything DCS, reading is what you'll do most. For me that is part of the fun.