A lot of captain decision making is common sense. Aviate, Navigate, Communicate. If the checklist says land at the nearest suitable airport, do that. Otherwise you will be sending messages to dispatch and maintenance through ACARS, or get patched through to the doctor on call, and will get input from them and from your FO on most inflight problems. CRM is important. Yes, the captain is ultimately responsible, but you are part of a team and will have a lot of help. You will also have a lot of training a long the way. Ask a lot of questions during IOE. No need to be afraid of upgrade.