On the technical side, learn how to read other people's code and debug things. At the junior level, when you thing things are broken, they're probably not, but the skills you build reading and proving that the other stuff works will come in handy as you progress and you do find situations where the frameworks you use, and the languages you use, and the operating systems you use are broken.
Learn to tear down abstractions instead of building them up. As a junior developer, it may be hard to get acceptance for removing layers, but add as few as possible. Somebody is going to come through later and need to understand what you've done, when the stack is deep, that's much harder.
Don't trust other people's code to do what the readme says. If anything claims it's lightweight, count the lines of code and see. If you use other people's code and it's broken, it's going to hurt your users and you're responsible.
On a people skills level, learn how to disagree without being disagreeable. Invite people to share their reasons for things with real questions, and listen to their answers -- while they're sharing you can discuss why to do it their way vs the way you want to do it. This is a good practice for when you are the senior person instructing the junior person as well. Often the senior person is going to make the decision, but having a discussion about it can make it easier to accept.
Don't do this for everything though. Choose your battles, don't be the person who fights everything; unless you're getting good results and people are happy changing everything.