Listen more than you speak. You'll learn more that way.
Realize that as you're being trained as a critical care nurse, you are still a nurse, and responsible for knowing what you're doing. That means, if you don't understand why a med or procedure is ordered, ask, and if it doesn't make sense, keep asking. I realize not all workplaces will support you in this, so you do have a heavy responsibility to educate yourself on your own time so you CAN pick things up as quickly as possible. But sometimes doctors make mistakes in orders, or the patient's condition changes since the order was written, and if you don't understand what's going on you won't be able to head off a problem. It might not be your fault, exactly, but still, you're often the last set of eyes and ears at the bedside, the last safeguard. If something doesn't make sense to you, it might not be because you just don't know. It might be because you DO know, and something is wrong.
Follow policies. Don't be a hero. Keep your backside covered. Don't overinvest your emotions in your work. Example: our policy is now that pharmacy mixes medication drips unless there's a life-threatening emergent need for nursing to do it. A coworker had a patient on an insulin drip, had been calling pharmacy for a couple of hours that the bag would run dry at such-and-such a time, and of course, that time arrived and she didn't have her new bag of insulin. She was so invested in those blood sugars, she had spent alllllll day titrating the drip, and was furious that it was all going to go to waste. She charged off to the med room, stating she was going to mix it herself. I threw myself in front of the med room door and talked her down. Insulin is NOT that kind of emergency. Half an hour with no insulin drip would not threaten the patient's life. God forbid, something went wrong with the insulin drip, and they see that it was something she mixed herself, she'd be hung out to dry, whether it was her fault or not. Another example: a different coworker, once again deeply invested in an insulin drip, let herself feel rushed in hanging a new bag. I wasn't there, I don't know the whole story, but she wound up spiking the insulin onto the antibiotic piggyback tubing and running it very briefly at the antibiotic rate. This happened because she did not follow the policy of doublechecking new bags of dangerous drips with another nurse, because no one was available right that minute and she let herself think it was a crisis. She realized her mistake very quickly, and hollered for help and 'fessed up with no attempt to cover it up, and learned from it, and the patient was okay, so all was well. But it was scary.