How early can I check in for my flight online?

Hello there, I'm just wondering about something regarding my upcoming flight. Could you please clarify for me the earliest time that I can check in for my flight online? I'd like to make sure I'm doing everything as efficiently as possible and don't miss out on any advantages of checking in early. Also, do I need to bring any specific documents or information with me when I arrive at the airport after checking in online? Thank you for your help!