There are tons of things to do when you are a junior and people here here are suggesting a lot of great things, but if there is a single advise that I would give to a junior it would be:
Read code. See how people have solve a problem and how the solution was formed in order to be Solid and testable. Then when you write code try to implement a thing or two be yourself.
Bruno Souza (Java Champion) has given me this advice and I keep doing it ever since.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.