I guess I'm good fit to answer this question - (although, only Jon Skeet would be the perfect fit, if at all he's on quora).
So, if you start today, I must say it would be bit difficult to find questions worth answering on SO, for the level of questions has started to degrade. And the questions which are worth answering are either easy enough that almost everyone would know, or it's tricky enough to require a SME to answer that.
So, answering a question on SO is really about two things:
- How you race up with fellow users to answer relatively easy questions first.
- Concentrate on answering questions that very few would know of.
Let's cover them one by one. For the first point:
1. First thing which you would like to do is to maintain favorite tags, and ignored tags. You can set them on the home page itself -Stack Overflow.
2. Then open each of your favorite tags in separate tabs. I am generally more active on Java tag. So, I keep a tab opened as - Newest Java Questions. Advantage of this is, everytime a new questions comes up in that tag, the tab will show you that in parenthesis like this: (3) Newest 'java' questions. So you don't have to check every time for new questions. You'll come to know.
3. Once you find a question of your liking, you need to answer that as quick as possible. Now, having quite a whole lot of experience, I can say that, don't try to write the entire answer at once. Just write a couple of lines initially (but that should be correct), and submit the answer. And then keep on editing the answer until it is perfect. That ensures that your answer comes as the first on the answered list. And such answers have high probability to get more up-votes.
Moving on the 2nd point (answering expert level questions):
1. It's pretty simple. If you're expert on some concepts, say - `concurrency`, you again add it to favourite tag, open it in new tab, and browse through all the un-answered questions there.
2. Now, every tag has a "featured" tag, that contains questions with bounty. Answering those questions will get you extra points apart from regular up-votes point. Those are the questions that are not answered properly by anyone, and probably you might give some better insights.
But as I said, it's difficult to find questions worth answering now-a-days. Because, most of the beginner level questions would probably be closed as duplicates (as they would have certainly been answered before), or as of low-quality (where OP doesn't give enough details to the question). So, you've to be really very patient there.
P.S: Here's a warning: It can get addictive. And once it gets, you might spend 4-6 hours continuously in search of new answerable questions to gain reputations. Trust me, it will happen. That's the magic of gamification.