Biggest thing, ASK FOR HELP.
I see this a lot with juniors they come in and agree to everything and when they get stuck they panic and don't tell anyone. You are a junior you are not supposed to know much and the codebase will be brand new, especially since it's your first job. Nobody cares if you ask for help and it actually looks better than at the end of the day finding out you have been working on a problem already solved someone could of told you quickly. Everyone expects you to be there to learn just as much as you are to work.