Who doesn't have an opinion?
Maybe it has something to do with the company i keep, or with the fact that anyone can post anything on the web (it's like a giant global conversation where people can say anything they like and not worry about looking stupid), but it seems like everyone has strong opinions on everything... lest they be called a fence-sitter.
You pick an opinion, and defend it with everything you have - ad hominem, straw man arguments, ignoratio elenchi, whatever you have to do. No matter what, though, there is always a loser: knowledge.
Try saying "i was wrong". It's really damn hard - but you will be wrong from time to time. It takes balls to man up and say "you're right". I have a lot of respect for anyone who has an open enough mind, and who cares enough about knowledge, to bow out of an argument when they realise their position is flawed.
And if you're not confident about your sources - if you haven't satisfied your own standards of research - it's ok to say "i don't know for sure". Nothing spreads bad information faster than people rabidly arguing their half-baked opinions to everyone they talk to.
Debate for fun, debate for sport, but keep your mind open.