I think this is great platform and I do get answers to what I post. I however am confused about how do I start contributing by answering some of the questions. I am wondering how people find out the questions that they think they can answer. Do you keep going through all the questions? Are there any filters which says I am basic/moderate/advanced user in some topics?
I suggest you use the Favorite Tags and Ignored Tags feature on the SO front page to make sure you see only questions you feel qualified to answer.
Don't try to answer questions unless you're sure about the answer.
I just look at the front page and see what's there. :)
You can look for questions in areas you're good at. Click on a tag in the "Unanswered Tags" list in the right side of the screen in https://stackoverflow.com/unanswered/tagged/?page=5&tab=newest
In addition to what pavium has suggested, I subscribe to the RSS feed of the tags. It helps me keep my interaction within my Email client, which have RSS reader.