Hi there! Congrats on finishing your book. That's always a huge step when you love writing. This is a great community, you're going to like it here. There's plenty of people who are willing to help you become the best and most successful writer you can be. Its really exciting.
I know everyone has already said it before, but it bears repeating: get to know everyone. Take some time to post in the forums. Read ALL the FAQs. They're not only helpful, they're very useful. You'd be amazed.
Once you hit that fifty post mark you can start sharing your work. That will give you an idea of what kind of critique works well for you and what doesn't. It will give you plenty of time to learn the personalities of the people who frequent the boards. Then you can start looking for a beta. I would also suggest working with other people's work too...not only does it make you and integral part of the community, but it will help you become more aware of your strengths and weaknesses as a writer. That, too, will make it easier to find and destroy those pesky mistakes that keep us from being our best.
Good luck!