Yer.advice.is.reely.good.A good number have that response, yes. Of that group, a large number are like that because they've experienced a pitiful excuse of Christianity. If you're going to "witness" on the internet, you must acknowledge that its a battle ground of ideas only. To that end you need to be able to challenge people's philosophies and present philosophies that are more consistent and more valuable. This will never be achieved by first assuming Christianity is (i) obvious and (ii) true, then pleading to those assumptions. An example of this is "Christianity is right because the Bible tells us Jesus is Lord".
That kind of knowledge isn't going to come overnight, so its time to put some serious study in. A good place to start is Timothy Keller's "The Reason for God". He outlines the most popular and most compelling philosophical arguments against both theism and Christianity and demonstrates in layman terms their inconsistencies. Keller is excellent reading to prepare you for the kinds of highly thought out arguments you'll encounter. Next is Ravi Zacharia, who's probably one of the foremost and compelling speakers in the world on the objectivity of moral standards and their pointing to a God. After that I'd read NT Wright's "Simply Christian" which describes the "hints" within reality that point to a God, specifically the God of Christianity. While doing this, be reading everything by CS Lewis, who was once an atheist himself and put substantial thought into answering their toughest questions (especially "the Problem of Pain")
Beyond lots and lots of research the only other thing you can do is avoid certain mistakes.
1) Do not suffer temptation to retaliate when they insult you.
2) Do not, like most other "evangelist" simply assume that these people are fools and treat them accordingly.
3) Do NOT answer unless you can honestly say that you've considered their question carefully.