The biggest tip I learned as a beginner on this awesome site is this:
Don't be afraid, be respectful.
In other words, your opinion matters. How you view something or what you feel about something is uniquely yours and it is not wrong. So, don't be afraid to voice your opinion and don't be afraid if someone contradicts you. Instead, respectfully listen to the thoughts and opinions of others. You might be surprised by what you learn--or what you can teach.
The writing biz is complicated. Every day I learn something new and I've been at this for a long time. In fact, one of the most valuable things I've learned about my writing came from a peer so new to the process she hadn't even finished her own novel yet.
Don't be afraid. If you have a question ask it. If you have an opinion share it.
Be respectful. Treat others as you would like them to treat you. If you want a critique, give one. If you disagree with commentary, listen to it then ignore it.
In the process, you'll learn great things, meet great people and be a part of the best writing community on the web.
Oh yeah, and if you write for kids, PM me and I'll provide a password so you can learn a little more about the nuances of the juvenile biz. We've got great members who can help answer questions and address concerns specific to this age group.