At the completion of training when we become full fledged first officers, we satisfy the bare minimums for the legal requirement of a copilot. The captains we fly with understand and should be patient with us as are skills are only just developing, but the imperative is on us, we must get better.
During training, you are going to study exactly what they tell you to study. Once training is over, it's time for your own study regiment to catchup on what you missed and what wasn't covered.
I found myself personally weak on memorization, so I put lots of time into flashcards. I create flashcard decks with Anki and I highly recommend it. I fly four different aircraft types, so I can sort or mix the deck. In addition to limitations I have included Part 121 rules and other knowledge I need memorized.
My company's training was weak on systems. Therefore I studied systems in the weeks and months after initial.
I started at my airline as a 1500 CFI. I was a first officer at an airline where the captains had only 1000hrs of airline experience. Therefore I studied things that would give me experience. I read pilot biographies, I read lots and lots of accident reports. I can tell you endless specific examples where these stories and case studies transferred directly to decision making at my new job.
The following is sequence of progression through the various flying skills as an airline pilot. This the progression I felt myself go through. While this list might have slightly different order for you, most of these skills build upon each other.
At the time of writings I am still a first officer approaching 1,000hrs. I am absolutely proficient through step 8. Step 9 is where the work is. I can do it, but I find it challenging. Step 10 I haven't started practicing yet.