New to Competitive but not 40k itself

So due to a medical issue placing me off work for a year I've decided I want to finally delve into the 40k competitive scene.

Outside of learning my units inside and out. What would you say is important to be doing? (I usually play Dark Angels or Imperial Guard) Do people have cheat sheets for their army? I assume they'd help with mental fatigue. Just looking for advice to start. Nervous to start this journey..but excited as well.